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PRINCIPLES AND PRACTICE 

OF 

ELECTRICAL ENGINEERING 



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PRINCIPLES AND PRACTICE 

OF 

ELECTRICAL ENGINEERING 



BY 
ALEXANDER GRAY 

Whit. Sch., B. Sc. (Edin. and V McGill) 

ASSISTANT PROFESSOR OF ELECTRICAL ENGINEERING, 

MCGILL UNIVERSITY, MONTREAL, CANADA; 

AUTHOR OF ELECTRICAL MACHINE DESIGN 



First Edition 



McGRAW-HILL BOOK COMPANY, Inc. 
239 WEST 39TH STREET, NEW YORK 

6 BOUVERIE STREET, LONDON, E. C. 

1914 



THK6 



Copyright, 1914, by the 
McGraw-Hill Book Company, Inc. 



THE. MAPLE. PRESS. YORK- PA 

OCT 20 1914 

©CI.A387135 



PREFACE 

The following work is based on a lecture and laboratory course 
given to the senior civil, mechanical, and mining students at 
McGill University. It is therefore suited for men who desire 
to obtain a broad idea of the principles and practice of electrical 
engineering and who have only a limited amount of time to spend 
on the subject. For such men it is necessary to emphasize the 
fundamental principles, and to develop the subject by elaborating 
on these principles rather than by the solution of mathematical 
equations, because only in this way can the student be given such 
a grip of the subject in the short time available, that he is able 
thereafter to make intelligent use of the data contained in the 
electrical handbooks, or take up with advantage a further study 
of the special treatises on the subject. 

The book gives a self-contained lecture and laboratory course. 
The chapters on the control and applications of electrical ma- 
chinery have been so written that large sections of these chapters 
may be set for private reading. In the laboratory course, com- 
plete references are given to the theory and purpose of each ex- 
periment, and these references in no case go beyond the text 
contained in the body of the work. 

The author wishes to acknowledge his indebtedness to Mr. 
A. M. S. Boyd and to Mr. R. Kraus for their help and criticism. 

A. G. 

McGill University, 
Sept. 1, 1914. 



CONTENTS 



Preface v 

Introduction xxi 

CHAPTER I 

Magnetism and Magnetic Units 
Article Page 

1. Magnets 1 

2. Coulomb's Law 1 

3. The Magnetic Field 1 

4. Lines of Force 2 

5. Lines of Force from a Unit Pole 3 

CHAPTER II 

Electromagnetism 

6. Direction of an Electric Current 4 

7. Magnetic Field Surrounding a Conductor Carrying Current ... 4 

8. Force at the Center of a Circular Loop Carrying Current ... 4 

9. Electromagnets 5 

10. Force on a Conductor Carrying Current in a Magnetic Field . . 6 

11. Moving Coil Ammeters 8 

CHAPTER III 
Electromagnetic Induction 

12. Electromagnetic Induction 9 

13. The Direction of the Induced Electromotive Force 9 

14. Mutual Induction 11 

15. Self Induction . 11 

CHAPTER IV 
Work and Power 

16. Transformation of Mechanical into Electrical Energy 13 

17. Unit of Work 14 

18. Heat Energy and Electrical Energy 15 

19. Conversion Factors 15 

20. Problems on Work and Power 15 

vii 



vin CONTENTS 

CHAPTER V 

Electric Circuits and Resistance 
Article Page 

21. The Flow of Electricity 18 

22. Ammeters and Voltmeters 19 

23. Resistance Circuits 19 

24. Ohm's Law 20 

25. Specific Resistance 20 

26. Variation of Resistance with Temperature 21 

27. Power Expended in a Resistance 21 

28. Insulating Materials 22 

29. Dielectric Strength of Insulating Material 22 

30. Series and Parallel Circuits 22 

31. Voltage Drop in a Transmission Line 23 

CHAPTER VI 

Rheostats and Resistors 

32. Rheostats 25 

33. Resistors '. . 26 

34. Heater Units 27 

35. Cast-iron Grid Resistance 27 

36. Carbon Pile Rheostat 29 

37. Liquid Rheostat : 29 

38. Size of a Rheostat • 31 

CHAPTER VII 
Magnetic Circuits and Magnetic Properties of Iron 

39. Magnetic Field due to a Solenoid 32 

40. Permeability 33 

41. Reluctance of a Magnetic Circuit 33 

42. Magnetization Curves ' 34 

43. Residual Magnetism 35 

44. Molecular Theory of Magnetism 36 

45. Hysteresis 36 

CHAPTER VIII 
Solenoids and Electromagnets 

46. Pull of Solenoids 37 

47. Electric Hammer 38 

48. Variation of the Pull of a Solenoid 38 

49. Circuit Breaker 39 

50. Laws of Magnetic Pull 40 

51. Solenoids with Long and Short Plungers 41 

52. Iron-clad Solenoids 41 



CONTENTS ix 

Article Page 

53. Lifting and Holding Magnets 43 

54. Saturation of a Magnetic Circuit 46 

55. Electromagnetic Brakes and Clutches 46 

56. Magnetic Separator 47 

CHAPTER IX 
Armatuee Windings for Direct-current Machinery 

57. Principle of Operation of the Electric Generator 48 

58. Gramme Ring Winding 48 

59. Commutator and Brushes 50 

60. Multipolar Windings .' 51 

61. Drum Windings 52 

62. Lamination of the Armature Core 55 

CHAPTER X 
Construction and Excitation of Direct-current Machines 

63. Multipolar Construction 56 

64. Armature Construction 58 

65. Commutator 58 

66. The Brushes 58 

67. Poles and Yoke . 58 

68. Large Generators 58 

69. Excitation 59 

CHAPTER XI 

Theory of Commutation 

70. Commutation 62 

71. Theory of Commutation 63 

72. Shifting of the Brushes 63 

73. Interpole Machines 65 

74. Carbon Brushes 66 

CHAPTER XII 
Armature Reaction 

75. The Cross-magnetizing Effect 67 

76. The Demagnetizing Effect 68 

77. Effect of Armature Reaction on Commutation 68 

CHAPTER XIII 

Characteristics of Direct-current Generators 

78. Magnetization or No-load Saturation Curve 70 

79. Self Excitation 71 



x CONTENTS 

Article Page 

80. Regulation Curve of a Separately Excited or of a Magneto 

Generator 71 

81. Regulation Curve of a Shunt Generator 72 

82. To Maintain the Terminal Voltage Constant 74 

83. Compound Generators 74 

84. The Regulation Curve of a Series Generator 75 

85. Problem on Generator Characteristics 76 

CHAPTER XIV 
Theory of Operation of Direct-current Motors 

86. Driving Force of a Motor 78 

87. Driving and Retarding Forces in Generators and Motors .... 78 

88. The Back E.M.F " 79 

89. Theory of Motor Operation 80 

90. Speed and Torque Formula? 82 

91. Improvement of Commutation by Shifting of the Brushes. ... 83 

92. Armature Reaction in Generators and Motors 83 

CHAPTER XV 

Characteristics of Direct-current Motors 

93. The Starting Torque 85 

94. The Starting Resistance 85 

95. Motor Starter 87 

96. No-voltage Release 87 

97. Load Characteristics 88 

98. Effect of Armature Reaction on the Speed 89 

99. Variable Speed Operation 89 

100. The Starting Torque 90 

101. The Starting Resistance 91 

102. Load Characteristics 92 

103. Speed Adjustment 92 

104. The Compound Motor . . 93 

CHAPTER XVI 
Losses, Efficiency and Heating 

105. Mechanical Losses in Electrical Machinery 95 

106. Copper Losses 95 

107. Hysteresis Loss 95 

108. Eddy Current Loss 96 

109. Stray Loss 96 

110. The Efficiency of a Machine 97 

111. Heating of Electrical Machinery 99 

112. Permissible Temperature Rise 99 



CONTENTS xi 

CHAPTER XVII 

Motor Applications 
Article Page 

113. Limits of Output 100 

114. Open, Semi-enclosed and Totally Enclosed Motors 100 

115. Intermittent Ratings 101 

116. Effect of Speed on the Cost of a Motor 101 

117. Choice of Type of Motor ..101 

118. Line Shaft Drive 102 

119. Wood-working Machinery 102 

120. Reciprocating Pumps 103 

121. Traction Motors 103 

122. Crane Motors . '. 103 

123. Express Passenger Elevators . 103 

124. Shears and Punch Presses 103 

CHAPTER XVIII 

Adjustable Speed Operation of Direct-current Motors 

125. Speed Variation of Shunt Motors by Armature Control .... 105 
'126. Speed Variation of Shunt Motors by Field Control 107 

127. Speed Regulation of an Adjustable Speed Shunt Motor 107 

128. Electric Drive for Lathes and Boring Mills 108 

129. Multiple Voltage Systems 109 

130. Ward Leonard System 110 

131. Drive for Ventilating Fans Ill 

132. Armature Resistance for Speed Reduction 112 

133. Motors for Small Desk Fans 112 

134. Printing Presses 113 

CHAPTER XIX 
Hand-operated Face Plate Starters and Controllers 

135. Knife Switches 114 

136. Auxiliary Carbon Contacts 114 

137. Blow-out Coils 115 

138. Horn Gaps 116 

139. Fuses 117 

140. Circuit Breakers 117 

141. Motor Starters 117 

142. The Sliding Contact Type of Starter 117 

143. Starting Resistance 118 

144. Overload Release 118 

145. Multiple Switch Starters 119 

146. Compound Starters 120 

147. Speed Regulators 121 

148. Controllers for Series Motors 122 



xii CONTENTS 

CHAPTER XX 

Drum Type Controllers 

Article Page 

149. Drum Type Controllers 124 

150. No-voltage and Overload Release 125 

151. Street Car Controller for Series Parallel Control 126 

152. Reversing Drum . . . . , 129 

153. Mechanical Features of Drum Controllers 129 

CHAPTER XXI 

Automatic Starters and Controllers 

154. Automatic Solenoid Starter 130 

155. Float Switch Control 130 

156. Magnetic Switch Controller • . . . 131 

157. Multiple Unit Control of Railway Motors . 133 

158. Automatic Magnetic Switch Starters 133 

159. Automatic Starter with Series Switches 135 

CHAPTER XXII 

Electrolysis and Batteries 

160. Electrolysis ■. 139 

161. Voltameter 139 

162. Electric Battery 140 

163. Theory of Battery Operation 140 

164. Polarization 141 

165. The E.M.F. and Resistance of Cells # 141 

166. The Daniell Cell 141 

167. Calculation of the E.M.F. of a Daniell Cell 142 

168. Local Action 143 

169. Leclanche Cell 143 

170. Dry Cells . 143 

171. Edison Lalande Cell 144 

172. Power and Energy of a Battery 144 

173. Battery Connections 144 

CHAPTER XXIII 
Storage Batteries 

174. Action of the Lead Cell 146 

175. Storage or Secondary Battery 147 

176. Sulphation 147 

177. Construction of the Plates 148 

178. Construction of a Lead Battery 149 

179. Voltage of a Lead Battery 151 

180. Capacity of a Cell 153 



CONTENTS xm 

Article Page 

181. Ampere-hour Efficiency 153 

182. Watt-hour Efficiency 154 

183. Effect of Temperature on the Capacity 155 

184. Limit of Discharge 155 

185. Treatment of Lead Cells 156 

186. Action of the Edison Battery 157 

187. Construction of the Plates 158 

188. Construction of an Edison Battery 158 

189. The Voltage of an Edison Battery 160 

190. Characteristics of an Edison Battery 160 

CHAPTER XXIV 

Operation of Generators 

191. Operation of the Same Shunt Machine as a Generator or as a Motor 162 

192. Loading Back Tests 163 

193. Parallel Operation .164 

194. Shunt Generators in Parallel 164 

195. Division of Load among Shunt Generators in Parallel 165 

196. Compound Generators in Parallel . 166 

197. Division of Load among Compound Generators 167 

CHAPTER XXV 
Operation of Generators and Batteries in Parallel 

198. Isolated Lighting Plants 169 

199. Lighting Plants for Farm Houses . . 169 

200. Lamp Circuit Regulator 170 

201. Small Isolated Power Stations 171 

202. Resistance Control 171 

203. End Cell Control 172 

204. Booster Charge, End Cell Discharge 173 

205. Capacity of Battery 175 

206. Batteries for Rapidly Fluctuating Loads 176 

207. The Differential Booster 176 

208. Carbon Pile Regulator 176 

209. Floating Batteries 179 

CHAPTER XXVI 

Car Lighting and Variable Speed Generators 

210. Systems of Vehicle Lighting 181 

211. Straight Storage for Trains 181 

212. Head and End System . . ■ 181 

213. Carbon Pile Lamp Regulator 182 

214. The Axle Generator Systems 182 

215. Automatic Switch 183 



xiv CONTENTS 



Article ' Page 

216. Generator Regulator 183 

217. Pole Changer 184 

218. The Stone Generator 185 

219. Lighting Generators for Motor Cars 186 

220. Constant Speed Generators 186 

221. Bucking Field Coils 186 

222. Vibrating Contact Regulator 187 

223. The Rosenberg Generator 188 



CHAPTER XXVII 

Alternating Voltages and Currents 

224. The Simple Alternator 191 

225. The Wave Form . . 193 

226. The Oscillograph 193 

227. Frequency 194 

228. Vibrating Reed Type of Frequency Meter 195 

229. Average Value of Current and Voltage 197 

230. The Heating Effect of an Alternating Current 197 

231. Symbols 198 

232. Voltmeters and Ammeters for Alternating-current Circuits . . . 198 

CHAPTER XXVIII 
Representation of Alternating Currents and Voltages 

233 200 

234. Electrical Degrees 200 

235. Vector Representation of Alternating Voltages and Currents . . 201 

236. The Sum of Two Alternating Voltages of the Same Frequency . . 203 

CHAPTER XXIX 
Inductive Circuits 

237. Inductance , 205 

238. Make and Break Spark Ignition 206 

239. The Coefficient of Self Induction 206 

240. Alternating Currents in Inductive Circuits 207 

241. Voltage and Current Relations 208 

242. Power in an Inductive Circuit 209 

243. Examples of Inductive and Non-inductive Circuits 209 

244. Voltage, Current and Power in Resistance Circuits 211 

245. Resistance and Inductance in Series 212 

246. The Power Factor 213 

247. The Wattmeter 214 

248. Transmission Line Regulation and Losses 215 

249. Resistance and Inductance in Parallel 216 



CONTENTS xv 

CHAPTER XXX 

Capacity Circuits 

Article Page 

250. Condensers 218 

251. Capacity Circuits with Direct and with Alternating Currents . . 219 

252. Phase Relation between Voltage and Current in Capacity Circuits . 220 

253. Voltage and Current Relations in Capacity Circuits 221 

254. Parallel Plate Condenser 222 

255. Power in Capacity Circuits 223 

256. The Formulae Used in Circuit Problems 223 

257. Resistance, Inductance and Capacity in Series 224 

258. Resistance, Inductance and Capacity in Parallel 226 

CHAPTER XXXI 
Alternators 

259. Alternator Construction 229 

260. Two-phase Alternator 230 

261. Three-phase Alternators 231 

262. Y-Connection 233 

263. Delta-connection 233 

264. Voltages, Currents and Power in a Y-Connected Machine . . . 235 
265^ Voltages, Currents and Power in a Delta-connected Machine . . 236 

266. Connection of a Three-phase Load , 237 

267. Power Measurement in Polyphase Circuits 238 

268. Alternator Construction 239 

269. The Revolving Armature Type of Alternator 240 

270. The Inductor Alternator 241 

271. Magneto Alternators . 242 

CHAPTER XXXII 

Alternator Characteristics 

272. Armature Reaction 244 

273. Vector Diagram at Full-load 245 

274. Regulation Curves of an Alternator 245 

275.- Experimental Determination of Alternator Reactance 246 

276. Automatic Regulators 249 

277. Efficiency 250 

278. Rating of Alternators 251 

CHAPTER XXXIII 
Synchronous Motors and Parallel Operation 

279. Principle of Operation of Synchronous Motors 252 

280. The Back E.M.F. of a Synchronous Motor 253 

281. Mechanical Analogy 254 



xvi CONTENTS 

Article Page 

282. Vector Diagram for a Synchronous Motor . . . i 254 

283. Maximum Output 255 

284. Operation of a Synchronous Motor when Under- and Over-excited 256 

285. Use of the Synchronous Motor for Power Factor Correction . . 256 

286. Synchronizing 258 

287. Hunting 7 258 

288. Parallel Operation of Alternators 259 

CHAPTER XXXIV 

Transformer Characteristics 

289. The Transformer 261 

290. Constant Potential Transformer 261 

291. Vector Diagram for a Transformer 263 

292. Induction Furnace. 263 

293. Leakage Reactance 264 

294. Leakage Reactance in Standard Transformers and in Induction 

Furnaces 266 

295. The Constant-current Transformer 267 

296. The Efficiency of a Transformer 268 

297. Hysteresis Loss 269 

298. Eddy Current Loss 269 

299. Iron Losses '. 269 

300. The All-day Efficiency 269 

301. Cooling of Transformers 270 

CHAPTER XXXV 

Transformer Connections 

302. Lighting Transformers 273 

303. Connections to a Two-phase Line 273 

304. Connections to a Three-phase Line 276 

305. Advantages and Disadvantages of the Y- and Delta-connection . 278 

306. Types of Transformer 279 

307. The Autotransformer 279 

308. Boosting Transformers and Feeder Regulators 281 

CHAPTER XXXVI 
Polyphase Induction Motors 

309. The Induction Motor 283 

310. The Revolving Field 284 

311. The Revolving Field of a Three-phase Motor 285 

312. Multipolar Machines 287 

313. The Starting Torque 287 

314. The Wound Rotor Motor 288 



CONTENTS xvn 

Article Page 

315. Running Conditions 290 

316. Vector Diagrams for the Induction Motor 291 

317. Adjustable Speed Operation 293 

318. Induction Generator 294 

319. Self- starting Synchronous Motors 294 

320. Dampers for Synchronous Machines 295 

CHAPTER XXXVII 
Induction Motor Applications and Control 

321. Choice of Type of Motor 296 

322. Line Shaft Drive 297 

323. Wood-working Machinery 297 

324. Cement Mills 297 

325. Motors for Textile Machinery 298 

326. Adjustable Speed Motors 298 

327. Crane Motors 298 

328. Shears and Punch Presses 299 

329. Adjustable Speed Service 299 

330. Resistance for Adjustable Speed Motors 300 

331. Switches for Alternating-current Circuits 300 

332. Starting of Squirrel-cage Induction Motors 301 

333. Starting Compensator 302 

334. The Star-delta Method of Starting 304 

335. Starter for a Wound Rotor Motor ... 305 

336. Automatic Starters 306 

CHAPTER XXXVIII 

Single-phase Motors 

337. Single-phase Induction Motors 308 

338. Split-phase Method of Starting 308 

339. Running Torque of a Single-phase Motor 308 

340. Single-phase Series Motor 310 

341. Armature Reaction 312 

342. The Repulsion Motor 313 

343. Commutation of Series and Repulsion Motors 314 

344. Wagner Single-phase Motor 314 

CHAPTER XXXIX 

MOTOR-GENERATOR SETS AND ROTARY CONVERTERS 

345. Motor-generator Set 315 

346. The Booster Set 315 

347. The Balancer Set 316 

348. Three-wire Generator 317 

349. To Transform from Alternating to Direct Current 318 



xviii CONTENTS 

Article Page 

350. Rotary Converter 318 

351. Motor-generator Sets and Rotary Converters 319 

352. Polyphase Rotary Converter 320 

353. Split-pole Rotary Converter 320 

354. Frequency Changers 321 

CHAPTER XL 

Electric Traction 

355. Tractive Effort 322 

356. Speed Time Curve 323 

357. Energy Required by a Car 326 

358. Characteristics Desired in Railway Motors 327 

359. Motor Construction 329 

360. Distribution to the Cars 329 

361. Alternating- and Direct-current Traction 329 

362. Motor Car Trains 332 

363. Electric Locomotives 332 

364. Crane and Hoist Motors 332 

365. Braking 333 

366. Flywheel Motor-generator Sets for Mine Hoisting 335 

367. Safety Devices 337 

CHAPTER XLI 
Transmission and Distribution 

368. Direct-current Stations 338 

369. Alternating-current Stations 339 

370. The Voltages Used in Practice 341 

371. Comparison between Single-phase and Three-phase Transmission. 341 

372. Lightning Arresters 343 

373. Switches 345 

374. Overhead Line Construction 346 

375. Underground Construction 347 

376. Switchboards . 348 

377. Instrument Transformers 351 

CHAPTER XLII 
Electric Lighting 

378. The Carbon Incandescent Lamp 352 

379. The Tungsten Lamp . . . 352 

380. Gas-filled Tungsten Lamp "... 353 

381. The Unit of Light 354 

382. Arc Lamps .354 

383. The Direct-current Open Arc 355 

384. Direct-current Enclosed Arc 355 



CONTENTS xix 

Article Page 

385. Alternating-current Enclosed Arc 356 

386. Flame Arc Lamps 356 

387. Luminous Arc Lamp 357 

388. Mercury Vapor Converter 357 

389. Mercury Vapor Lamp 359 

390. Shades and Reflectors 359 

391. Efficiency of Illuminants 360 

392. Light and Sensation 361 

393. Reflection and Color 362 

394. Principles of Illumination 362 

395. Quality of the Light 363 

396. Glare 363 

397. Shadows 363 

398. Intensity of Illumination 364 

399. Lines of Illumination 364 

400. Power Distribution for Lighting 365 

CHAPTER XLIII 
Laboratory Course 

401. Protection of Circuits 368 

402. Ammeter Shunts 368 

403. Safe Carrying Capacity of Copper Wires 368 

404. Control of the Current in a Circuit 369 

Exp. 1. Measurement of the Resistance of the Field Coil Circuit . . . 370 

Exp. 2. Measurement of the Resistance of the Armature Circuit . . 370 

Exp. 3. Speed Adjustment of a Direct-current Shunt Motor .... 371 

Exp. 4. Voltage of a Direct-current Generator . 371 

Exp. 5. Regulation of Direct-current Generators 372 

Exp. 6. Brake Tests on Direct-current Motors . 373 

Exp. 7. Starting Torque Tests on Direct-current Motors 373 

Exp. 8. Stray Loss and Efficiency of a Direct-current Motor .... 374 

Exp. 9. Heat Run on a Direct-current Generator 374 

Exp. 10. Voltage Regulation of a Three-wire System 374 

Exp. 11. Fuse Testing 375 

Exp. 12. Calibration of a Circuit Breaker 375 

Exp. 13. Alternating-current Series Circuit 376 

Exp. 14. Predetermination of the Characteristics of an Alternating 

Current Circuit 376 

Exp. 15. Characteristics of a Constant Potential Transformer .... 377 

Exp. 16. Regulation of an Alternator 377 

Exp. 17. Starting and Running Characteristics of a Synchronous 

Motor 378 

Exp. 18. Characteristics of a Rotary Converter 379 

Exp. 19. Starting and Running Characteristics of a Polyphase Induc- 
tion Motor 380 

Exp. 20. Transformer Connections 381 

Index . 383 



INTRODUCTORY 

Before a study of electric circuits and machinery can be 
made, it is necessary to define the electric and the magnetic 
units and express them in terms of the fundamental units and 
derived mechanical units which are given below. 

Quantity Practical units Practical units 

c.g.s. system ft. lb. sec. system 

Length 1 cm. 1 f t = 30.48 cm. 

Mass 1 gm. 1 lb = 453.6 gm. 

Time 1 sec. 1 sec. 

Force 1 dyne 1 poundal = 1/32.2 lb. 

1 gm. = 981dynes 1 lb. = 4.448 X 10 5 dynes 
Work or 

energy 1 erg = 1 dyne cm. 1 ft. lb. = 1.356 X 10 7 ergs 
Power 1 erg per sec. 1 h.p. = 550 ft. lb. per sec. 

= 746 X 10 7 ergs per sec. 

In the first few chapters of this work some of the fundamental 
principles of electricity and magnetism are briefly discussed. 
Parts of these chapters are difficult and are of theoretical im- 
portance only. These are printed in small type and may be 
omitted if the student is willing to consider as experimental 
laws what are really laws depending on the definitions of the 
electric and the magnetic units and on their interrelations. 



XXI 



PRINCIPLES AND PRACTICE 

OF 

ELECTRICAL ENGINEERING 

CHAPTER I 
MAGNETISM AND MAGNETIC UNITS 

1. Magnets. — The power of a magnet to attract or repel is 
concentrated at certain points called poles. A simple magnet 
has two poles which are equal and opposite and the line joining 
them points north and south when the magnet is allowed to 
swing freely in a horizontal plane. The pole pointing toward 
the north is called the north (N) pole, that pointing toward 
the south is called the south (S) pole. 

Like poles repel one another, unlike poles attract one another. 

2. Coulomb's Law states that the force between two magnetic poles is 
directly proportional to the strengths of the poles and inversely proportional 
to the square of the distance between the poles, thus, in Fig. 1, 

m m\ 

# * # 

Fig. 1. 

mm i 



/ = &- 



where / is the force between the poles, 

r is the distance between the poles, 

m and mi are the strengths of the poles, 

k is a constant which depends on the surrounding medium and on the 

units chosen. 

The c.g.s. unit of pole strength is chosen so as to make k = 1 when/ is in 

,. ,. rami 

dynes, r m cm. and the medium is air, then / = . 

r 2 

A unit pole therefore acts on an equal pole in air, at a distance 
of 1 cm. from it, with a force of 1 dyne. 

3. The magnetic field is the name given to the space surround- 
ing a magnet, but is limited in practice to the space within which 
the force of the magnet is perceptible. A magnetic pole placed 
in a magnetic field is acted on by a force which is proportional 

1 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, i 



to the strength of the magnetic pole and to the strength or in- 
tensity of the magnetic field. 

The intensity of a magnetic field at any point is taken as the 
force in dynes on a unit pole at that point; therefore, a unit field 
will act on a unit pole in air with a force of 1 dyne. 

The direction of a magnetic field at any point is taken as the 
direction of the force on a north pole at that point. 




Fig. 2. — Direction of the field of a magnet. 

Let NS, Fig. 2, be a magnet of pole strength m, and n a unit north pole. 
The pole N of the magnet repels the unit pole with a force = ra/ri 2 dynes, 
represented in magnitude and direction by the line na; the pole S of the mag- 
net attracts the unit pole with a force = m/r 2 2 dynes, represented in magni- 
tude and direction by the line nb ; the resultant force on the unit pole, which 
is a measure of the field intensity, is represented in magnitude and direction 
by the line nc. 




Fig. 3. — Lines of force surrounding a bar magnet. 

4. Lines of Force. — In dealing with magnetic problems it 
is found convenient to represent the magnetic field diagram- 
matically by what are called lines of force. These are con- 
tinuous lines whose direction at any point in the field is that of 
the force on a north pole placed at the given point. The number 
of lines crossing 1 sq. cm. placed perpendicularly to this direction 
is made proportional to the field intensity at the point and unit 
magnetic field is represented by one line per sq. cm. 



Art. 5] 



MAGNETISM AND MAGNETIC UNITS 



In Fig. 3 the intensity of the magnetic field is greatest at the 
poles and decreases as the distance from the poles increases, so 
that the lines of force which represent this field spread out from 
the poles as shown. Since a north pole n placed in this field is 
repelled by the pole N and attracted by the pole S, the lines of 
force, being drawn in the direction of 
the force on a north pole placed in the 
field, must leave the N pole and enter 
the S pole. 

The total number of lines of force 
leaving or entering a magnetic pole is 
called its magnetic flux <J>. 

The flux density (B at any point in 
a magnetic field is the number of lines 
of force crossing unit area placed per- 
pendicular to the direction of the lines 
of force at that point. 




Fig. 4. 



5. Lines of Force from a Unit Pole. — If a unit pole were surrounded by a 
sphere of 1 cm. radius, as in Fig. 4, another unit pole placed on the surface of 
this sphere would be acted on with unit force and so the field intensity at 
this surface must be unity; there must therefore be one line of force per 
sq. cm. of sphere surface or a total of 4?r lines, as the surface area of a sphere 
of 1 cm. radius is 4t sq. cm. 

Since the number of lines from a unit pole is 4x, therefore the number from 
a pole of strength m is 4n-m. 



CHAPTER II 
ELECTROMAGNETISM 

6. Direction of an Electric Current— P and Q, Fig. 5, are 
conductors carrying current; the current is going down in con- 
ductor P and coming up in conductor Q. Let the direction 
of the current be represented by an arrow; at the end of con- 
ductor P one would see the tail of the arrow, represented by a 
cross, while at the end of conductor Q the point of the arrow 
would be seen, this is represented by a dot. 




Out In 

Fig. 5. — Direction of an electric current. 



7. Magnetic Field Surrounding a Conductor Carrying Current. 
— A conductor carrying current is surrounded by a magnetic 
field represented by lines of force as shown in Fig. 6. To deter- 




Current Down. Current Up 

Fig. 6. — Field surrounding a conductor carrying current. 

mine the direction of these lines the following rule is used: If 
a corkscrew is screwed into the conductor in the direction of 
the current then the head of the corkscrew has to be turned in 
the direction of the lines of force. 

8. Force at the Centre of a Circular Loop Carrying Current. — Fig. 7 shows 
a wire, carrying a current i, and bent to form a circular loop of radius r. 
The direction of the magnetic field produced is found by the rule in the last 
paragraph. 

4 



Art. 9] 



ELECTROMAGNETISM 



An element ab acts on a unit pole at the centre of the loop with a force / 



which is found to be = k 
complete loop 



abXi 



and the total force F on this pole due to the 



2tv rXi 



= k 



2tt% 



where k is a constant which depends on the medium and on the units chosen. 
The unit of current is chosen so as to make k = 1 when F is in dynes, r is 




Fig. 7. — Magnetic field produced by a loop carrying current. 

in cm. and the medium is air, then F = — dynes; a unit current is therefore 

of such value that, when flowing in a loop of 1 cm. radius, it acts on a unit 
pole at the centre of the loop with a force of 2t d}mes. This is called the 
e.g. s. unit of current; the practical unit, called the ampere, is equal to one- 
tenth of a c.g.s. unit. 

9. Electromagnets. — The loop carrying current, shown in 
Fig. 7, acts like a magnet and is called an electromagnet. The 




Fig. 8. — The polarity of an electromagnet. 

strength of an electromagnet may be increased by increasing 
the current or, as in Fig. 8, by increasing the number of turns. 
The direction of the magnetic field may be conveniently found by 
another corkscrew law which states that if the head of the work 
screw is turned in the direction of the current then the screw- 



6 PRINCIPLES OF ELECTRICAL ENGINEERING [Chaf.ii 

itself will move into the magnetic field in the direction of the lines 
of force; the direction of the field produced by the right-hand 
spiral in diagram A is the same as that produced by the left- 
hand spiral in diagram B; this direction may be reversed by 
reversing the current. 

10. Force on a Conductor Carrying Current in a Magnetic Field. — In 

Fig. 9, the unit po>le n is acted on by the current in the loop with a force of 




Fig. 9. — Force on a conductor carrying current in a magnetic field. 

2-iri/r dynes (page 5) at right angles to the plane of the paper, where i 
is the current in c.g.s. units. The loop itself must be reacted on by the unit 
pole with an equal force in the opposite direction. 

The flux density (B at the wire, in lines per sq. cm., due to the unit pole 

flux from the unit pole 
surface of a sphere of r cm. radius 
4tt _ 1 
4rrr 2 " r 2 

As shown above, the force acting on the wire in dynes 

_ 2?r i 
r 

= -j X 2irr X i 



= ($>Li. 



Since (B 



Art. 10] 



ELECTROMAGNETISM 



where (B is the flux density at the wire in lines per sq. cm., L is the length of 
wire that is in the magnetic field in cm. = 2-kt in the case of a circular loop 
i is the current in the wire in c.g.s. units. 

When a conductor is carrying current and is in a magnetic 
field, as in Fig. 10, it is acted on by a force which is proportional 




Fig. 10. — Force on a conductor carrying current in a magnetic field. 
Left Hand Ruel: thumb — force; forefinger — lines of force; middle finger — 
current. 

to the current and to the strength of the field. Tbe direction 
of this force may be determined by what is called the left-hand 
rule which states that if the thumb, the forefinger and the 




Fig. 11. — Moving coil ammeter. 

middle finger of the left hand are placed at right angles to one 
another as in Fig. 9 so as to represent three coordinates in space, 
with the thumb pointed in the direction of the mechanical force 
and the forefinger in the direction of the lines of force, then the 
middle finger will point in the direction of the current. 



8 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, ii 

11. Moving Coil Ammeters. — The above principle is applied 
in one of the most satisfactory types of instrument for the 
measurement of direct current. 

Such an instrument is shown in Fig. 11. NS'' is sl permanent 
horseshoe magnet with pole shoes bored out cylindrically and 
E is a cylindrical soft iron core concentric with the pole faces, 
lines of force therefore pass as shown in diagram A and the flux 
density in the air gaps is uniform. In this magnetic field a coil 
C is placed and is supported on jewelled bearings. The coil 
consists of a number of turns of fine insulated wire wound on a 
light aluminium frame and the current to be measured is intro- 
duced to the coil through the spiral springs D, diagram B. Since 
the sides of the coil are carrying current and are in a magnetic 
field they are acted on by forces which turn the coil through an 
angle against the torsion of the springs D and this angle may be 
read on a scale over which plays a pointer B attached to the 
coil. 



CHAPTER III 
ELECTROMAGNETIC INDUCTION 



12. Electromagnetic Induction. — Faraday's experiments 
showed that when the magnetic flux threading a coil undergoes a 
change, an electromotive force (e.m.f.) is generated or induced in 
the coil and that this e.m.f. is pro- 
portional to the time rate of change 
of the flux. If the coil A, Fig. 12, 
be moved from position 1 where the 
flux threading the coil is </> lines, to 
position 2 where the flu,x threading 
the coil is zero, in a time of t seconds, 
then the average rate of change of 
flux is (j)/t lines per sec. 

The c.g.s. unit of e.m.f. is that 
generated in a coil of one turn when 
the flux threading the coil is chang- 
ing at the rate of one line per sec. 
The practical unit, called the volt, 
is equal to 10 8 c.g.s. units so that 
when the flux threading a coil of one 
turn changes at the rate of 4> lines 
in t seconds the average e.m.f. in- 
duced in the coil — — 10 ~ 8 volts and 




the e.m.f. at any instant = -tt 10 
volts. 



Fig. 12. — Generation of elec- 
tromotive force. Right Hand 
Rule: thumb — motion; fore- 



That portion of the coil wherein the fiSpMiectromot^liorel^ 
e.m.f. is actually induced is the con- 
ductor xy which cuts the lines of force, and the quantity d<f>/dt is 
the rate at which the lines are cut. 

13. The direction of the induced electromotive force may 
be determined by Fleming's three-finger right-hand rule which 
states that if the thumb, the forefinger and the middle finger of 
the right hand be placed at right angles to one another so as to 

9 



10 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, hi 



represent three coordinates in space, with the thumb pointed 
in the direction of motion of the conductor relative to the 
magnetic field and the forefinger in the direction of the lines of 
force, then the middle finger will point in the direction of the 
induced e.m.f. 

The direction of the current in xy, as determined by the right-hand rule, is 
shown in Fig. 13 for the case where the coil is moving downward and the 
number of lines of force threading the coil is decreasing. This current sets 
up a magnetic flux <f> c , the direction of which, found by the corkscrew law 
(page 5) is the same as that of the main flux <f> and tends to prevent the flux 
threading the coil from decreasing. 




Fig. 13. Fig. 14. 

Direction of the generated electromotive force. 

If now the direction of motion of the coil be reversed so that the number of 
lines of force threading the coil is increasing, the current will be reversed, as 
shown in Fig. 14, and the magnetic flux <j> c will oppose the main flux and 
tend to prevent the flux threading the coil from increasing. 

The general law for the direction of the induced e.m.f. in a coil, 
known as Lenz's Law, states that the induced e.m.f. tends to 
send an electric current in such a direction as to oppose the change 
of flux which produces it. 

If the coil abed, Fig. 15, is moved from m to n, the flux threading 
the coil does not change and the resultant e.m.f. generated in the 
coil is zero; the portions ab and cd of the coil are cutting lines 
of force but the e.m.fs. generated in these portions are equal 
and opposite. 



Art. 15] 



ELECTROMAGNETIC INDUCTION 



11 



14. Mutual Induction. — The flux threading a coil may be 
changed without moving the coil. Suppose a constant current is 
flowing in the coil A, Fig. 16, this produces a constant flux 4> which 
threads coils A and 5 but noe.m.f. is generated in coil B since there 




Fig. 15. 

is no change in the flux. If the current in coil A is increased, the 
flux threading coil B will increase and this change of flux will in- 
duce an e.m.f . in coil B which will cause a current I 2 to flow in such 
a direction as to oppose the increase in flux. If the current in coil 
A is decreased, the flux threading coil B will decrease and this 
change of flux will induce in coil B an e.m.f. which will send a cur- 
rent 1 2 in such a direction as to oppose the decrease in flux. 






A 

L -fM v 






n 


B 




Flux is 


0" 


-i , 




J 2 , 


M 


' 


Increasing 


i\ 
















5 


sH"^ 




h 







p 


A 

p 


P 


P 


\ 


V 






Flux is 


' 


■ > 


• ' 


' ' 


- ' 


Is 




Decreasing 


/ 







T 


1 


/ 


J 


u 


k 


.2 




L 


u 






E 


a 






c 







Fig. 16. — Direction of electromotive force of mutual induction. 



15. Self Induction. — When the current in a coil is changed, an 
e.m.f. is generated in the coil itself in such a direction as to oppose 
the change in the current. In Fig. 17, for example, when the 
switch k is closed, the current flowing in the coil does not reach its 
final value instantaneously because, as the current increases in 
value, the flux <f> threading the coil increases and causes an e.m.f. 
to be induced in the coil in such a direction as to oppose the in- 



12 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, in 



crease of the current. This opposing e.m.f., called the e.m.f. of 
self induction, exists only while the current is changing. 

If, after the current has reached its final value, the switch k is 
suddenly opened, the current in the coil tries to decrease suddenly 
to zero but, as it decreases, the flux threading the coil decreases 
and causes an e.m.f. to be induced in the coil in such a direction 



yd 



+ 

Fig. 17. 



Applied Voltage 




Fig. 18. — Growth of current in a coil. 



as to oppose the decrease of the current; this e.m.f. is generally 
large enough to maintain the current between the switch contacts 
as they are being separated and accounts for much of the flashing 
that is seen when a switch is opened in a circuit carrying current. 
When the switch is closed, the current increases to its final value 
as shown in Fig. 18. As the current i increases, the correspond- 
ing increase of the flux <£ threading the coil induces an e.m.f. of 
self induction which is proportional to dcf>/dt, the rate of change 
of the flux. When the current has ceased to change, the e.m.f. 
of self induction becomes zero. 



CHAPTER IV 
WORK AND POWER 

16. Transformation of Mechanical into Electrical Energy. — 

If the conductor xy, Fig. 19, be moved downward so as to cut at 
a constant rate the lines of force passing from N to S, a constant 
e.m.f. is induced in the conductor and, by adjusting the resistance 
R, the current in the circuit may be maintained at the value i 




Fig. 19. 

Right Hand Rule for generation of e.m.f.: thumb — motion; forefinger — 
lines of force; middle finger — e.m.f. 

Left Hand Rule for direction of force: thumb — force on conductor; fore- 
finger — lines of force; middle finger — current. 

in the direction shown; the direction of the current may be de- 
termined by the right-hand rule (page 9). 

As this conductor is carrying current in a magnetic field, it is 
acted on by a force F the direction of which may be determined 
by the left-hand rule (page 7). This force, as shown in Fig. 19, 
opposes the motion of the conductor and hence mechanical 
energy must be expended in moving the conductor. 

13 



14 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, iv 

If (B is the density of the magnetic field in lines per sq. cm. 

L is the length in cm. of that part of the conductor which is cutting lines of 

force 
V is the velocity of the conductor in cm. per sec. 
i is the current in the conductor in c.g.s. units, then 
e, the e.m.f . generated in the conductor in c.g.s. units, 
= the lines of force cut per sec. 
= ®>LV 
Now F, the force acting on the conductor = (BLi dynes (page 6) and 
the mechanical power in dyne cm. per sec. required to keep the 
conductor moving 
= FV 
= ((BLi)7 
= (($>LV)i 
= ei 

= (volts X 10 8 ) (amperes/ 10); pages 9 and 5 
= volts X amperes X 10 7 

The mechanical power required to obtain I amperes at a 
difference of potential of E volts from an electrical machine which 
has an efficiency of 100 per cent. 

= EI 10 7 ergs per sec. 
= EI watts 

where the watt, the practical unit of power, is equal to 10 7 ergs 
per second. 

The power developed by large electrical machines is expressed 
in kilowatts, where 1 kw. is equal to 1000 watts. 

The horsepower = 550 ft. lb. per sec. 

= 746 X 10 7 ergs per sec. 
= 746 watts. 

this result gives a connecting link between the electrical and the 
mechanical units. 

17. Unit of Work. — Work is done when a force is moved 
through a distance. The c.g.s. unit of work is the erg, which 
is the work done in moving a force of 1 dyne through a distance 
of 1 cm. 

Power is the rate at which work is done and is expressed 
either in ergs per sec, in watts (10 7 ergs per sec), or in horse- 
power (746 X 10 7 ergs per sec). 

When the power, or rate at which work is being done, is 1 
watt, or 10 7 ergs per sec, then the work done in 1 sec 
is 1 watt-second or 10 7 ergs and is called 1 joule. A more 



Art. 20] WORK AND POWER 15 

convenient unit for practical work is the kilowatt-hour (3600 X 10 3 
joules) and this will gradually replace the horsepower-hour 
because it is based on a system of international units. 

18. Heat Energy and Electrical Energy. — The energy required 
to raise the temperature of 1 lb. of water by 1° F. is called 
the British Thermal Unit (B.T.U.) and is equal to 780 ft. lb. 
The energy required to raise 1 gm. of water through 1° C. 
is called the gramme calorie and is equal to 4.2 X 10 7 ergs so 
that 

1 gm. calorie = 4.2 X 10 7 ergs 

= 4.2 watt-seconds (joules). 

19. Conversion Factors. — Although the c.g.s. system of units 
is the only possible international system, much calculation 
work is still carried out in the foot-pound-second system. The 
conversion factors given below help to simplify the work of 
changing from one system to another. 

C.g.s. unit Other units 

Length 1 cm. 1 in. = 2.54 cm. 

Mass 1 gm. 1 lb. = 453.6 gm. 

Time 1 sec. 

Force 1 dyne 1 gm. = 981 dynes 

1 lb. = 444,800 dynes 
= 453.6 gm. 
Work or energy 1 erg = 1 dyne-cm. 1 joule = 1 watt-sec. 

10 7 ergs 
1 ft. lb. = 1.356 X 10 7 ergs 
1 kw.-hour = 3600 X 10 3 joules 
1 gm. calorie = 4.2 joules 
1 lb. calorie = 1900 joules 
Power 1 erg. per sec. 1 watt = 10 7 ergs per sec. ^ 

1 kw. = 1000 watts 
1 h.p. = 550 ft. lb. per sec. 
= 746 watts 

20. Problems on Work and Power. 

1. A hoist raises a weight of 2000 lb. through a distance of 300 ft. in a 
time of 1 min. Find the work done and the power expended. 

If the efficiency of the hoist is 75 per cent, and that of the motor is 90 per 
cent, find the horsepower of the motor and also the current taken by the 
motor if the voltage is 110. 

a. Work done = 2000 X 300 

= 600,000 ft. lb. 



16 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.iv 

b. Power expended = 600,000 ft. lb. per 60 sec. 

= 10,000 ft. lb. per sec. 

10,000 

18.2 h.p. 



550 
18.2 X 746 
1000 ~ 



13.6 kw. 



•toe 

c. The power input to the hoist = — ~ = 18.1 kw = 24.3 h.p. 

0.75 

18 1 
The power input to the motor = — — = 20 kw. 
F F 0.9 

d. Since watts = volts X amperes, therefore 20 X 1000 = 110 X amperes 
and the current in amperes = 182. 

2. An electric iron takes 5 amp. at 110 volts. What does it cost to operate 
this iron for 2 hours if the cost of energy is 6 cents per kw.-hour. 

The rate at which energy is used = 110 X 5 = 550 watts 

= 0.55 kw. 
The energy used in 2 hours = 0.55 X 2 = 1.1 kw.-hour 
The cost of this energy = 1.1 X 6 = 6.6 cents 

3. A 32-candle power, 110-volt tungsten lamp requires 40 watts. What is 

the current taken by this lamp and what is the cost of energy for 15 lamps 

burning for an average time of 4 hours if the cost of energy is 5 cents per 

kw.-hour? 

watts 40 

amperes = — - — = = 0.36 amp. 

1 volts 110 

power = 40 X 15 = 600 watts 

= 0.6 kw. 

energy used = 0.6 X 4 = 2.4 kw. -hours 

cost of energy = 2.4 X 5 = 12 cents. 

4. An electric water heater has an efficiency of 80 per cent, and takes 3 
amp. at 110 volts. How long will it take to raise 1 pint (1.25 lb.) of 
water from 20° C. to the boiling point and what will this cost when the 
rate is 5 cents per kw.-hour ? 

energ}^ required = 1.25 (100 — 20) = 100 lb. calories 

= 100 X 1900 = 190,000 watt-sec. 

100 
energy delivered = 190,000 X 

= 238,000 watt-sec. 
= 238 kw.-sec. 
= 0.066 kw.-hours 
cost of energy = 0.066 X 5 = 0.33 cents 

Now 238,000 watt-sec. are supplied at the rate of 110 X 3 = 330 watts 
therefore the time during which energy must be applied 

238,000 
= ' = 720 sec. 
330 

= 12 min. 



Art. 20] WORK AND POWER 17 

5. If a ton (2000 lb.) of coal heats a house for a month what would it cost 
to give exactly the same heating effect electrically if the cost of energy is 3 
cents per kw.-hour? 

With a good heating system 1 lb. of coal burnt on the grate will deliver 
8000 B.T.U. or 4450 lb. calories to the house. 

The energy required per month therefore = 4450 X 2000 lb. calories 

= 8,900,000 lb. calories. 
= 8,900,000 X 1900 watt-sec. 
8,900,000 X 1900 , , 

= ^r^ 7^7: kw.-hours 

3600 X 1000 

= 4,700 kw.-hours 
the cost of this energy = 4,700 X 3 = 14,100 cents 

= 141 dollars. 

The reason for this enormous difference in the cost of heating by the two 
methods is that the efficiency of a heating system is about 60 per cent, 
while that of an electric generating station is about 6 percent.; moreover 
the cost of the coal required per kw.-hour is about 0.5 cents or 1/6 of 
the selling price of the electrical energy. 



CHAPTER V 

ELECTRIC CIRCUITS AND RESISTANCE 

21. The flow of electricity through electric circuits is similar 
in many ways to the flow of water through hydraulic circuits. 
This may be seen by a comparison between the circuits shown 
diagrammatically in Fig. 20. 



Pump 





T B A 

I CTi.Voltmeter 




Fig. 20. — Hydraulic and electric circuits. 



To maintain a steady current 
of w gm. of water per sec. 
through the hydraulic circuit 
and to raise the water from b to 
a through a difference of poten- 
tial of h cm., an amount of power 
= wh gm. cm. per sec. must be 
put into the circuit by the 
pump. 

In returning from a to b 
through the external part of the 
circuit, the water falls through 
a difference of potential of h cm. 
and supplies an amount of 
power = wh gm. cm. per sec. to 
drive the turbine and to supply 
the frictional resistance loss in 
the pipes. 



To maintain a steady elec- 
tric current of I coulombs per 
sec. (amperes 1 ) through the 
electric circuit and to raise the 
electricity through a difference 
of potential of E volts, an 
amount of power = EI watts 
must be put into the circuit by 
the electric generator. 

In returning from a to b 
through the external part of the 
circuit, the electricity falls 
through a difference of poten- 
tial of E volts and supplies an 
amount of power = EI watts 
to drive the motor and to 
supply the resistance loss in the 
connecting wires. 



l A current of electricity is expressed in amperes; there is no corresponding 
unit for a current of water which must therefore be expressed in gm. cm. per 
sec. The quantity of electricity which passes any point in a circuit is ex- 
pressed in coulombs where 1 coulomb is 1 amp. -sec. A larger unit is the 
ampere-hour. 

18 



Art. 23] ELECTRIC CIRCUITS AND RESISTANCE 



19 



The current of water (the 
quantity passing any point per 
sec.) is the same at all points 
in the circuit since the circuit is 
closed. 



The electric current (the 
quantity passing any point per 
sec.) is the same at all points 
in the circuit since the circuit is 
closed. 



22. Ammeters and Voltmeters. — The current in a circuit may 
be measured by means of an instrument such as that described 
on page 8, connected directly in the circuit as shown at A, 
Fig. 20, while the difference of potential between two points 
may be measured by means of a similar instrument connected 
directly between the points as shown at B. The essential differ- 
ence between the two instruments is that the ammeter must 
carry the total current in the circuit with only a small difference 
of potential across its terminals and must therefore offer a 
small resistance to the flow of current through it, the volt- 
meter on the other hand must divert only a small portion of 
the current from the circuit and must therefore offer a large 
resistance to the flow of current through it. 



Pump. 





Fig. 21. — Hydraulic and electric circuits. 



23. Resistance Circuits. — Consider the case represented dia- 
grammatically in Fig. 21 where there is no turbine in the hydraulic 
circuit nor any motor in the electric circuit. 



The difference of potential of 
h cm. maintained by the pump 
is used up in forcing w gm. of 
water per sec. against the fric- 
tional resistance of the pipe, and 

h = wr 

where r is called the resistance 
of the pipe circuit. 



The difference of potential of 
E volts maintained by the elec- 
tric generator is used up in 
forcing I amperes against the 
resistance of the wires, and 

E = IR 

where R is called the resistance 
of the electric circuit and is a 
constant for a given circuit. 



20 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, v 

This resistance increases with This resistance increases 
the length and decreases with with the length and decreases 
the cross section of the pipe. with the cross section of the 

wire, or 

length 



R 



section 



24. Ohm's Law. — The above relation E = IR is known as 
Ohm's law and the unit of resistance, called the ohm, is chosen 
of such a value that a circuit with a resistance of 1 ohm will 
allow 1 amp. to flow when the difference of potential between 
the ends is 1 volt, therefore 

volts = amperes X ohms 

If for example the current in the heating coil of a 110-volt 
electric iron is 5 amp., then the resistance of this coil = 110/5 
= 22 ohms. 

25. Specific Resistance.— As pointed out in art. 23, the re- 
sistance of a wire is directly proportional to its length and 
inversely proportional to its cross section or 

where R is the resistance of the wire in ohms 

L is the length of the wire 

A is the cross section of the wire 

k is a constant called the specific resistance and depends 
on the material and on the units chosen. If centimeter units are 
used then the specific resistance is the resistance of a piece of 
the material 1 cm. long and 1 sq. cm. in cross section and is 
expressed in ohms per cm. cube. 

In practice the unit of cross section is generally taken as the 
circular mil which is defined as the cross section of a wire 1 mil 
(1/1000 in.) in diameter. 

Since a wire 1 mil in dia. has a section of 1 cir. mil a wire 1 
inch in dia. has a section of 10 6 cir. mils and a wire 1 sq. inch in 

4 

section has a section of 10 6 cir. mils. 

x 

The specific resistance of copper wire 1 is 1.6 X 10~ 6 ohms per 
cm. cube or 9.7 ohms per cir. mil foot at 0° C; the specific 

1 For values of specific resistance of various materials see Standard Hand- 
book for Electrical Engineers. 



Art. 27] ELECTRIC CIRCUITS AND RESISTANCE 21 

resistance of cast iron is 80 X 10 -6 ohms per cm. cube or 480 
ohms per cir. mil foot, approx., at 0° C. 

26. Variation of Resistance with Temperature. — The resist- 
ance of most materials varies with the temperature and 

R t = R (l + at) 

where R t is the resistance at t° C. 

R is the resistance at 0° C. * 

t is the temperature of the* material in deg. C. 

a is called the temperature coefficient of resistance. 

For all pure metals the resistance increases with the tempera- 
ture and a is approximately equal to 0.004. The resistance of 
carbon and of liquid conductors decreases with increase of tem- 
perature, while the resistance of special alloys such as manganin 
remains approximately constant at all operating temperatures. 

A coil has 1000 turns of copper wire with a cross section of 1288 cir. mils 
and a length of mean turn of 15 in. 

a. Find the resistance of the coil at 0° C. 

b. Find the resistance of the coil at 25° C. 

c. Find the current that will flow through the coil at 25° C. and 110 volts. 

d. After current has passed through the coil for some time it is found that 
its value has dropped to 9 amp., find the average temperature of the coil 
under these conditions. 

a. The resistance of 1 cir. mil foot = 9.7 ohms at 0° C. 

9.7 X 1000 X 15 

The resistance of the coil at 0° C, = — = 9.4 ohms. 

1288 X 12 

b. The resistance of the coil at 25° C. = 9.4 (1 + 0.004 X 25) = 10.3 ohms. 

no 

c. Ihe current = = 10.7 amp. 

10.3 P 

d. The hot resistance of the coil = = 12.2 ohms at t° C. 

9 

The resistance of the coil also = 9 A ohms at 0° C. 
Therefore the resistance of the coil at t° C. = 9.4(1 + 0.0040 

= 12.2 ohms 
from which 1 + 0.004i = 12.2/9.4 = 1.3 

and t = 0.3/0.004 = 75° C. 

27. Power Expended in a Resistance. — To force a current of 
i" amperes through a circuit which has a resistance of R ohms, a 
voltage E = IR is required so that 

the power expended in the circuit = EI watts 

= (IR)I 

= PR watts. 
This power is transformed into heat. 



22 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, v 



In the electric flat iron and other such heating apparatus this 
heat is utilized, the heating element consisting of a coil of high 
resistance wire, insulated with heat resisting insulation such as 
asbestos, or mica, and embedded in the iron. 

28. Insulating materials are materials which offer a very large 
resistance to the flow of electric current and for that reason they 
are used to keep the current in its proper path. In a transmission 
line, current is prevented from passing between the wires by 
porcelain insulators attached to cross arms as shown in Fig. 22. 
When the wires are placed close to one another, as in house wiring, 
they are covered throughout their entire length with insulating 
material such as paper, rubber or cotton; the electrical resistance 
of these materials is greater than 10 10 ohms per cm. cube. 




Porcelaiii,' \. 
Insulator P^~~~ ^^ 

/ , , \ 




Fig. 22. — Insulators for transmission lines. 



29. Dielectric Strength of Insulating Material. — If a sheet of 
insulating material is placed between two terminals and the 
voltage between the terminals is gradually raised the material 
will finally break down and a hole be burnt through it, the 
material is then said to be punctured, and a large current will flow 
through the puncture if the voltage is maintained. The property 
of an insulating material by virtue of which it resists breakdown 
is called its dielectric strength. 

30. Series and Parallel Circuits. — If several conductors are 
connected in series as shown in Fig. 23, then the current is the 



Art. 31] ELECTRIC CIRCUITS AND RESISTANCE 



23 



same in each conductor while the total voltage is the sum of the 
voltages across the different parts of the circuit so that 

E = Ei + E2 + E% -}- E \ 
= I{R l + R2 + R 3 + £ 4 ) 

If several conductors are connected in parallel as shown in Fig. 
24, then the voltage across each conductor is the same while the 
total current is the sum of the currents in the different paths so 
that 

I = h + I2 + h + h 

(1 J^ _1_ _1_" 
\Ri R2 R3 Ra 



WWWW- 

U—E-i-Ji, 



< Ez—>\ 



U 



-E*->\ 



R A I Rz 

Fig. 23. — Series circuit. 




Fig. 24. — Parallel circuit. 



Four coils having resistances of 3, 5, 10 and 12 ohms respectively are 
connected in series across 120 volts, find the current in the circuit and the 
voltage drop across each coil. 

120 = 7(3 + 5 + 10 + 12) 
= 307 
therefore 7=4 amp . 

Ei = 4: X 3 = 12 volts 
E 2 = 4 X 5 = 20 volts 
E z = 4 X 10 = 40 volts 
Ei = 4 X 12 = 48 volts 

If these coils are now connected in parallel across 120 volts, find the current 
in each coil and also the total current. 

7i = 120/3 = 40 amp. 
h = 120/5 = 24 amp. 

7 3 = 120/10 = 12 amp. 

7 4 = 120/12 = 10 amp. 
total cm-rent 7 =86 amp. 

31. Voltage Drop in a Transmission Line. — When electric 
energy is transmitted from one point to another over wires, a 



24 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.v 



voltage, called the drop in the line, is required to force the cur- 
rent through the wires. This voltage = IR where R is the total 
resistance of the connecting wires and J is the current flowing, 
so that if Eg, Fig. 25, is the voltage at the generating station, and 
E r is the voltage applied to the load in the receiving station, 
then E g = E r + IR. 




Fig. 25. 



The power to be delivered at the end of a 2 mile line is 30 kw. If the 

receiver voltage is 600, find the size of wire required to limit the voltage drop 

in the line to 5 per cent., find also the power loss in the line. 

30 X 1000 rn 

= 50 amp. 

600 



Current in line = 



The voltage drop in the line = 5 per cent, of 600 = 30 volts 
The resistance of the wire in the line = 30/50 = 0.6 ohms 
The resistance of copper = 9.7 ohms per cir. mil foot at 0° C. 
= 9.7 (1 + 0.004 X 25) 
= 10.6 ohms per cir. mil foot at 25° O. 
The resistance of 2 miles of line or 4 miles of wire 
. 10.6 X 4 X 5280 
cir. mils 
From which cir. mils = 370,000 
The loss in the line = 30 volts X 50 amp. 
= 1500 watts 
= 5 per cent, of the power delivered. 



= 0.6 ohms 



CHAPTER VI 
RHEOSTATS AND RESISTORS 

32. Rheostats. — A rheostat is an adjustable resistance of 
such a form that it can be conveniently used. In the rheostat 
shown in Figs. 26 and 27, the resistance ab is tapped at eight 




Fig. 26. 




Fig. 27. — Sliding contact type of rheostat. 

points which are connected to contact studs s over which the 
handle H is free to move. 

Such a rheostat is used to control the current in a circuit. 

25 



26 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, vi 



When the handle is in the position shown in Fig. 26, all the re- 
sistance ab is in the circuit and the current I has its minimum 
value. When the handle is in the position Hi, current flows 
through the path TcdebQ so that the resistance between a and 
e has been cut out. As the handle is moved further over, the 
resistance in the circuit is further decreased until finally, when 
the handle is in the position Hi, the resistance is all cut out and 
the current in the circuit has its maximum value. 

33. Resistors. — For economy in manufacture, resistances such 
as ab, Fig. 26, are generally built up of standard resistance 




Fig. 30. 




Fig. 28. 



Porcelain Tube 




Fig. 29. 

Resistance units. 



Fig. 31. 



units called resistors, which may be connected in series or in 
parallel as desired. Different types of resistance units are shown 
in Figs. 28 to 33. 

The unit shown in Fig. 28 consists of a length of wire wound 
on an iron tube, from which it is insulated by fireproof insulation 
such as asbestos. 

In Fig. 29 a similar unit is shown which consists of a length of 
wire wound in a spiral groove cut on the surface of a tube of 
porcelain or some other such material, adjoining turns of the 
wire being thereby separated from one another. 

Such units are mounted in frames as shown in Fig. 27. They 
are always placed vertically so that air can circulate freely 
through the tubes and over the surface of the resistance wire 
and thereby keep the temperature of the rheostat within 
reasonble limits. 

For carrying comparatively small currents, round wire is 



Art. 35] 



RHEOSTATS AND RESISTORS 



27 



suitable; strip metal is preferred for larger currents, as it gives 
a larger surface for a given section. An excellent type of con- 
struction is shown in Fig. 32 where the resistance unit consists 
of a length of resistance strip metal wound on a frame consisting 
of an iron plate A insulated at the edges with porcelain sup- 
porting pieces B. These units may be mounted on iron rods 
which pass through the holes C. 

34. Heater Units. — Fig. 30 shows the external appearance 
of a type of resistor which is largely used for electric irons 
and other such heating appliances. It is constructed of re- 
sistance strip wound in the form of a helix and placed in a metal 
tube which is lined with mica, the tube is then packed with fire- 



fi 



W 




Fig. 32. — Resistance unit. 



proof cement to insulate adjacent turns from one another, and 
the open end of the tube is closed with a cement plug through 
which the leading in wires are brought. 

Another type of heater unit is shown in Fig. 31 and consists 
of a length of resistance wire wound into a helix of small diameter, 
which helix is then coiled into a flat spiral and mounted in a 
frame with mica between the convolutions. This unit is held 
against a layer of quartz grains which are embedded in enamel on 
the bottom of the heater. 

35. Cast-iron Grid Resistance. — When large currents have to be 
controlled, the necessary cross section to carry the current and the 
necessary radiating surface to dissipate the heat are'best obtained 
by the use of zig-zag units of the shape shown in Fig. 33. For 
small rheostats, these zig-zag pieces may be punched out of sheet 
metal, but for larger sizes they are generally of cast iron as 
shown in Fig. 34. 

The method or assembling these castings is shown in Fig. 35 
which is a plan of a rheostat similar to that in Fig. 34. The units 



28 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, vi 



A are mounted on iron rods B which are insulated throughout their 
entire length by mica or asbestos tubes C. The individual units 



o 


















o 



Fig. 33. — Zig-zag resistance unit. 




Fig. 34. — Cast-iron grid resistance. 



D Metal Washer 

F Insulating Washer 
AlicaTube 






Fig. 35. — Flow of current in 
a grid resistance. 



Fig. 36. — Carbon pile rheostat. 



are separated from one another by washers which are either of 
metal as at D and E or of insulating materials as at F, depending 



Art. 37] 



RHEOSTATS AND RESISTANCE 



29 



on the direction in which it is desired to make the current flow. 
The four metal washers E act as terminals from which leads 
can be taken to the contacts on the control faceplate. 

36. Carbon Pile Rheostat. — An entirely different type of rheo- 
stat is shown in Fig. 36 and consists of a column of graphite discs 
A , enclosed in a steel tube B which is lined with fireproof insula- 
tion such as asbestos. The resistance of such a pile decreases as 
the mechanical pressure between the ends increases, because the 
contact between adjacent discs improves. In the type of rheo- 
stat shown in Fig. 36 the pressure is applied by turning the hand- 
wheel D and is communicated to the carbon pile through the 
plungers E. The two units shown may be connected in series or 
in parallel as desired, and the resistance of such a rheostat can be 
changed gradually through a total range of about 100 to 1. 




Fig. 37. — Liquid rheostat. 

37. Liquid Rheostats. — Such a rheostat is shown in Fig. 37 
and consists of a cast-iron trough A which contains a solution of 
caustic soda or some similar material which does not attack iron, 
and an iron plate B which is insulated from the tank as shown at E 
and which dips into the liquid. Between the terminals Ti and T 2 
therefore there is the resistance of the path through the liquid 
between A and B, and the section of this path can be increased or 
decreased by lowering or raising the plate B. The resistance may 
be finally short circuited by lowering B far enough to allow the 
contact H to close, then current can pass direct from T\ to T% 
without passing through the liquid. 

Another type of liquid rheostat is shown in Figs. 38 and 39. 



30 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, vi 



In this case the plates are fixed but the level of the liquid is 
varied. The pump D sends a continuous stream of liquid from 
the cooling chamber A through the resistance chamber B, and 
the level of the liquid in this latter chamber may be raised 
or. lowered by a weir C. 




Fig. 38. 




Fig. 39. — Liquid rheostat. 

The liquid is cooled in the lower chamber by water which flows 
through cooling pipes. This cooling chamber sometimes takes the 
form of a concrete tank made large enough to allow the rheostat 
to be self cooling. 



Art. 38] RHEOSTATS AND RESISTORS 31 

Liquid rheostats are largely used in making load acceptance 
tests on generators. Two electrodes in a barrel of water in which 
a handful of common salt has been dissolved will dissipate about 5 
kw. if the water is stationary. The type of temporary rheostat 
most generally used however consists of a bank of cast-iron grids 
mounted in a wooden frame and placed in running water, the grids 
will carry about four times as much current under these conditions 
as when air cooled. 1 

38. The size of a rheostat depends principally on the amount 
of power which it is required to dissipate. If two rheostats have 
to dissipate the same amount of power but one has only half as 
much current flowing as the other then, since the loss in the 
rheostat = PR watts, the former rheostat must have four times 
the resistance of the latter, that is the wire must have half the 
section and twice the length, but the weight of wire and the space 
occupied by the rheostat will be approximately the same in each 
case. 

1 For design data on such temporary rheostats see the Standard Handbook 
for Electrical Engineers. 



CHAPTER VII 

MAGNETIC CIRCUITS AND MAGNETIC PROPERTIES 

OF IRON 

39. Magnetic Field due to a Solenoid. — A solenoid is a coil of 
wire wound in the form of a helix as shown in Fig. 8, page 5. 
When an electric current is passed through such a coil it acts as 
an electromagnet and the direction of the magnetic field may be 
found by the corkscrew law, page 5. 



A Sq.Cm. 




T Turns 



Fig. 40. — Closed solenoid. 

The solenoid in Fig. 40 has T turns wound on a cardboard spool and is 
bent to form an annular ring. A current of i c.g.s. units flowing through 
these T turns produces a magnetic field of intensity 3C which field can 
therefore be represented by 5C lines of force. 

If a unit pole n be moved once round the magnetic circuit through a dis- 
tance of 2-kt centimeters in a time of t seconds then, since the force on this 
pole due to the electromagnet is JC dynes, the work done in moving the 
pole = 5C X 2-n-r ergs. But a unit pole has 4-n- lines of force, see page 3, 
and while this pole is moved once around the magnetic circuit these lines 
cut the T turns of the coil in a time of t seconds and generate in the coil an 
e.m.f. e, which in c.g.s. units = ^irT/t, the number of lines cut per second. 
The coil therefore acts as a generator and supplies an amount of power = ei 
ergs per second so long as the unit pole is moving, that is for a time of t 
seconds. This power must be obtained at the expense of the power ex- 
pended in keeping the unit pole moving so that 

32 



Aet. 41] MAGNETIC PROPERTIES OF IRON 33 

3C X 2tt r = eit ergs 

= (4nT/t)it 

= 4*Ti 

Ti 
therefore 3C = 4?r y- where i is in c.g.s. units and L = 2irr 

4*TI , r . . 
= Yn "T~ where i is m amperes. 

In a magnetic circuit such as that shown in Fig. 40 the field 
intensity 5C is given by the formula 

no - ^-Tl 

X ~ 10 L 

where T is the current in amperes 

T is the number of turns of the solenoid 

TI is called the ampere-turns 

L is the length of the magnetic circuit in cm. = 27rr in the 

above case 
5C is the field intensity in the magnetic circuit and is also 
the flux density or the number of lines of force per sq. 
cm. of solenoid cross section. 
The total magnetic flux threading the magnetic circuit is 

= 3CA 

_ ^lTIa 

- 10 L A 

where A is the cross section of the solenoid in sq. cm. 

40. Permeability. — If the solenoid is wound on a core of mag- 
netic material such as iron or steel it is found that for the same 
number of exciting ampere-turns a much larger magnetic flux is 
produced and that 

4tt TI 

(B, the flux density = -^ ~r~ M lines per sq. cm. 

4>, the magnetic flux = y^ -y- A n lines of magnetic flux. 

where ^ is a quantity called the permeability of the material and 
is equal to unity for air and is greater than unity for magnetic 
materials such as iron and steel. 

41. Reluctance of a Magnetic Circuit. — The above general law 
for the magnetic circuit may be expressed in slightly different form 
namely 

" io TI x T 



34 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, vii 

m.m.f. 

~ (R 
or m.m.f. = <£(R 

where m.m.f. called the magnetomotive force, is that which 

produces the magnetic flux and = wkTI ampere-turns 

<f> is the number of lines of magnetic flux in the magnetic circuit 

(R called the reluctance of the magnetic circuit = - -j 

From its similarity to the law for the electric circuit, namely 
e.m.f. = IR, the above law is sometimes called Ohm's law for 
the magnetic circuit. 

Since the permeability of iron is much greater than that of air, 
the reluctance of an iron path is much lower than that of an air 
path of the same dimensions. 



18 
16 

i» 

6- 
™ 12 

p. 
1 10 

•S 8 



2 6 

Q 

I 4 

















• | 1 


. 


































S^^rrZ 


































e^TSteel 




















B 




























































































































































!\ 






































































































































s? 


Tt 


(y» 






































































































































B 
/ 








































/ 






































/ 








































/ 









































10 20 30 40 50 60 70 80 
Ampere Turns per Cm. 

Fig. 41. — Magnetization curves. 



100 



42. Magnetization Curves. — The magnetic properties of iron 
and steel are generally shown by means of magnetization curves 
such as those in Fig. 41; the data from which these curves are 
plotted is determined in the following way. 

Test pieces of iron are made in the form of an annular ring 
with a cross section of A sq. cm. and a mean length of magnetic 
path of L cm. These rings are then wound uniformly with T 
turns of wire as in Fig. 40 and the flux <j> is measured for different 
values of the current I by means of special instruments. 



Akt. 43] 



MAGNETIC PROPERTIES OF IRON 



35 



The value of <f>/A, the flux density, is then plotted against 
corresponding values of TI/L, the ampere-turns per unit length 
of magnetic path, as shown in Fig. 41, to give what is called the 
magnetization curve of the material. 

The permeability /* = Tpfir X j_ may then be determined. 

When this value is plotted against flux density, as in Fig. 42, 
it may be seen that, once a particular density has been reached, 
the permeability decreases rapidly with increase of flux density. 
Permeability curves are seldom used in practice, it is found to 
be more convenient to work with magnetization curves such as 



2000 

1800 

1600 

1400 
>, 

3 1200 
a 
1 1000 

800 

600 

400 

200 


















































































\ 


> 


















K% 


































































fis 






























i 



2 4 6 8 10 12 14 16 18x10 8 
Flux Density in Lines per Sq. Cm. 

Fig. 42. — Permeability curves/ 

those shown in Fig. 41. An example of the use of such curves 
is given on page 44. 

43. Residual Magnetism. — If, after a piece of iron has been 
magnetized by means of an exciting coil, the exciting current is 
reduced to zero, it will be found that the magnetism has not be- 
come zero but that some of it, called the residual magnetism, 
remains. If the iron is soft and annealed, this residual magnetism 
will be of negligible amount and the last traces of it may be made 
to disappear if the iron is subjected to vibration. If hard tool 
steel is used the residual magnetic field will be strong and the 
residual magnetism can be removed only with difficulty so that 
permanent magnets are generally made of this material. 



36 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, vii 

44. Molecular Theory of Magnetism. — To account for the 
peculiar magnetic behavior of iron, Ewing suggested that mole- 
cules of iron are natural magnets each with its own north and 
south pole. When the iron does not exhibit magnetic properties 
then the molecular magnets are arranged in groups as shown in 
diagram A, Fig. 43, and their magnetic effects neutralize each 
other. 

If the iron is placed in a strong magnetic field the molecular 
magnets will turn and point in the direction of the field as shown 
in diagram B, Fig. 43. 

If a piece of iron is placed in an exciting coil, a small current in 
this coil will turn these molecular magnets which are not strongly 
held together and will line them up in the direction of the mag- 



«=> C=- ^ C=*» ^ ^_ , 



A not }Iagnetized B ilagnetized 

Fig. 43. — Arrangement of the molecules of an iron bar. 

netizing force, these magnets will then add their own magnetic 
flux to that which the coil would produce if no iron were present. 
As the exciting current is increased, more of these magnets are 
lined up until, when the point B has been reached on the curve 
in Fig. 41 all but the most rigid of the molecular magnets have 
been lined up and the magnetic flux can then increase but little 
even for a large increase in the excitation. 

When the exciting current is reduced to zero and the magnetiz- 
ing force thereby removed, the molecular magnets reform into 
groups but, on account of molecular friction, they do not return 
quite to their original position but have a slight permanent dis- 
placement in the direction in which they have been magnetized 
arid this accounts for the residual magnetism. 

45. Hysteresis. — There is another phenomenon in connection 
with the magnetization of iron which can readily be explained 
by the molecular magnet theory, namely, that if the magnetism 
of a piece of iron is reversed rapidly the iron becomes hot. What 
is called hysteresis energy has to be expended in overcoming the 
molecular friction of the magnets and this appears in the form of 
heat. 



CHAPTER VIII 
SOLENOIDS AND ELECTROMAGNETS 

46. Pull of Solenoids. — A solenoid is a conductor wound in 
the form of a helix. When an electric current is passed round a 
solenoid a magnetic field is produced, the direction of which may 
be determined by the corkscrew law, page 5. This field may 
be represented by lines of force as shown in diagram A, Fig. 44. 



7Hm 



m 



w 



i,V\ 



7/ 

v J 






Fig. 44. — Action of a solenoid. 



If long bar magnets are placed in the solenoid field as shown in 
diagram B, Fig. 44, then the n pole of magnet x will tend 'to move 
in the direction of the lines of force, see page 2, and be pulled 
into the solenoid, while the s pole of magnet y will tend to move 
in a direction opposite to that of the lines of force so that it also 
tends to move into the solenoid. 

If the current in the solenoid is reversed, the magnetic field of 
the solenoid will reverse and the magnets x and y will be repelled. 

37 



38 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, viii 



If as in diagram C, Fig. 44, soft iron plungers are used instead 
of bar magnets, then the lines of force produced by the solenoid 
will pass through the plungers and cause magnetic poles to be 
induced; north poles will be formed where the lines of force leave 
the iron and south poles where they enter, see page 3. The 
induced polarity of the plungers shown in diagram C is the same 
as the polarity of the bar magnets in diagram B so that the 
plungers are pulled into the solenoid. 

If the current in the solenoid is now reversed, the magnetic 
field of the solenoid will reverse but, since the induced polarity 
of the plungers will also reverse, the direction of the pull on the 
plungers will be unchanged. 

47. Electric Hammer. — The two types of electric hammer 
shown diagrammatically in Figs. 45 and 46 illustrate the action 
of a solenoid with a magnet plunger and with a soft iron plunger 
respectively. 








A 






B 








. 


, 




■n f f 




1 










^ 








. J 




J J J 


u 

1 








i 







Fig. 45. Fig. 46. 

Diagrammatic representation of electric hammers. 

. In Fig. 45, current passed through coil C makes the iron plunger 
into a magnet with the polarity as shown. If now a current I is 
passed through coils A and B in the direction indicated by the 
arrows, then the plunger p will be attracted by A and repelled 
by B and will move toward the left. If this current I is now re- 
versed, the plunger will move in the opposite direction, so that, 
by continually reversing the current that flows through A and B, 
the plunger p may be made to reciprocate. 

Another type of hammer is shown in Fig. 46. The soft iron 
plunger is pulled into the position shown when coil A is excited, 
while if coil B is excited the plunger is pulled into this latter 
coil. By alternately exciting the two coils, the plunger may be 
made to reciprocate. 

48. Variation of the Pull of a Solenoid. — When the plunger is 
in the position shown in A, Fig. 47, the reluctance of the magnetic 



Art. 49] 



SOLENOIDS AND ELECTROMAGNETS 



39 



circuit is large since the path of the lines of flux is nearly all 
through air, so that the magnetic field and the plunger poles are 
both weak. As the plunger moves toward F, the reluctance of 
the magnetic circuit decreases because the amount of iron in the 
magnetic path is increasing, so that the magnetic field and the 
plunger poles become stronger. 

With further motion of the plunger in the same direction, the 
reluctance of the magnetic circuit continues to decrease and the 





Pull of a solenoid. 



strengths of the magnetic field and of the plunger poles to 
increase, but the induced south pole of the plunger now begins 
to come under the influence of the soletioid field and is repelled 
so that, although the north pole is still attracted, the resultant 
pull decreases and finally becomes zero when the plunger is in the 
position shown in diagram B; the reluctance of the magnetic 
circuit has then its minimum value. 

The pull on the plunger varies with its position as shown in 
diagram C; over a considerable range the pull is constant. 

49. Circuit Breaker. — The variation in the pu,ll of a solenoid is 
taken advantage of in the type of circuit breaker shown dia- 
grammatically in Fig. 48. Such a circuit breaker consists of the 



40 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, viii 

switch C closed against the force of the spring S and held closed 
by the latch d. This latch is released by the plunger p which is 
lifted when the line current passing round the solenoid M reaches 
a predetermined value, the spring $ then forces the switch open. 
If the plunger p is moved further into the solenoid by means of 
the adjusting screw a then the current required to lift this plunger 
will be decreased, ©^ this means the circuit breaker can be 
adjusted to open with different currents. 




Solenoid M 




Fig. 48. — Automatic circuit 
breaker. 



Fig. 49. — Electromagnetic motor. 



50. Laws of Magnetic Pull. — The law of inverse squares, art. 
2, page 1, applies only to imaginary point magnets; in prac- 
tical work the following laws are applied. 

The force on a piece of iron in a magnetic field in air tends to 
move the iron in such a direction as to reduce the reluctance of 
the magnetic circuit. 

The magnitude of this force at any point is proportional to the 
space rate of change of the magnetic flux as the iron passes the 
given point. 

An interesting application of this rule is shown diagrammatic- 
ally in Fig. 49. The lines of force due to the coils A and B pass 
through the magnetic circuit as shown by the arrows and the 
pivoted piece of iron p tends to move until the reluctance of the 
magnetic path is a minimum, that is, until the air gaps between 
n and s have their minimum value and p is pointing in the 
direction ab. If the shape of the curved parts from a to b is such 
that the magnetic flux <£ increases uniformly with the angle turned 



Art. 51] 



SOLENOIDS AND ELECTROMAGNETS 



41 



through by p then the turning force, being proportional to the 
space rate of change of flux, will be constant over the whole 
range of motion. 

The above principle is frequently used in toy electromagnetic 
motors, provision being made for cutting off the current in the 
exciting coils when p approaches close to the position ab and for 
switching the current on again when this point is passed. 

51. Solenoids with Long and with Short Plungers. — When the 
plunger is of the same length as the solenoid, the pull becomes zero 
when the plunger is in the position shown in diagram B, Fig. 
47, the position of minimum reluctance. 





Distance X 

Fig. 50. — Pull of a solenoid. 



When the plunger is longer than the solenoid, as is generally 
the case in practice, the reluctance does not become a mini- 
mum until the plunger projects equally from both ends as 
shown in diagram B, Fig. 50, so that the range of the solenoid 
is increased, as may be seen by a comparison between the curves 
in Fig. 47 and Fig. .50. 

52. Ironclad Solenoids. — In order to reduce the reluctance of 
the return part of the magnetic circuit and at the same time to 
protect the windings, the ironclad construction shown in Fig. 
51 is used. When the plunger is in the position shown in diagram 
A, the reluctance of the magnetic circuit is nearly all in the air 
path ab. As the plunger moves toward b, the flux increases and 
changes very rapidly toward the end of the stroke so that, while 



42 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, vin 

the average pull is not much higher than that of the same solenoid 
with an air return path, a large pull over a short distance is ob- 




Simple Ironclad Type 





B 

Cushion Type 



.Stopped Jronclad Type 

Fig. 51. — Types of ironclad electromagnet. 

tained at the end of the stroke as shown in curve B, Fig. 52. 1 
If a hole for the plunger be bored through the iron cover as 




3 4 5 6 7 

i Distance X in Inches 

Fig. 52. — Pull of electromagnets. 



10 u 12 



shown in diagram B, Fig. 51, then there is no sudden jar at the end 
of the stroke but rather a cushion effect; the large increase of pull 

x Taken from an article by Underhill, Electrical World and Engineer, 
Vol. 45, p. 934 (1905.) 



Art. 53] 



SOLENOIDS AND ELECTROMAGNETS 



43 



at the end of the stroke is lost however, although this is seldom 
a disadvantage. 

Fig. 53 shows a series of test curves on ironclad magnets of 
the cushion type and will give the reader some idea of the magni- 
tude and range of pull that can be obtained. 



so < 


■ • , i 1 1 ; h i ■ li _ _ _ .. 


1 


|lll|il||lllllillil 






ie; 






! i Ml II 


1 1 1 1 




i ! ! , i 




«^i i j i | | NJ 


I ! ' ! : ! 1 III 




^_T^~> S ! i 


St 


: ; 


„ ^^nL ^v 1 


i\ II 1 1 | II Mil 


1 1 1 1 ! j 


^150 ' \V^ 








' \ 1 i 




15 k \ 






P-t | | | 1 \ > > ■ i 






a \ ^i 








*N. i i V ' ' 1 ' ' i 


I' 7 "" — 




iS, ^^ 


^— »^. i 


* "_. \ 


j ^s^i 'ill ^Ts^i 










pi 




i | 1 p ^^j_ i 1 |U — • 


n | 


\ \ 


■ 


50 ^""tti" 


^ ^ 




. X-L--1 


X I 




J_ 




I i 1 


_L 


. _ ^_ 




* T 


- 1 ~ 


1 1 i 



4 5 C 7 

Stroke in Inches 



10 11 12 



Fig. 53. — Pull of cushion type of electromagnet. 

53. Lifting and holding magnets are generally of the horse- 
shoe or of the annular type shown diagrammatically in Figs. 54 
and 55. As the iron to be lifted moves from a to b, Fig. 55, there 



\ I 





Fig. 54. — Horseshoe type of Fig. 55. — Annular type of 
electromagnet. electromagnet. 



is little change in the flux threading the coil M and therefore only 
a small pull; when the iron approaches close to the poles of the 
magnet, however, the flux increases rapidly and the pull, being 
proportional to the space rate of change of flux, becomes large. 



44 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, viii 

For such magnets the holding power may be determined very 
closely by Maxwell's formula 

pull in dynes = 

Sir 

where <B is the flux density across the contact surface in lines per 
sq. cm. 
A is the total pole face area in sq. cm. 

In the case of the magnet shown in Fig. 56, the scale on the iron to be lifted 
is assumed to be 0.05 cm. thick, it is required to determine how the pull varies 
with the exciting current. 

To solve this problem it is necessary to assume different values for the 
total flux in the magnetic circuit then calculate the pull by the use of the 
above formula and the excitation by the use of the curves in Fig. 41, page 34. 




Cast Iron 



1.0 2.0 3.0 

Amperes 



Fig. 56. — Pull of a horseshoe magnet. 



h, the length of the cast steel path = 20 cm. 
h, the length of each air gap = 0.05 cm. 
h, the length of the cast iron path = 12 cm. 
Ai, the cross section of the cast steel path = 2 X 4 = 8 sq. cm. 
A 2 , the cross section of each air path = 8 sq. cm. 
A 3 , the cross section of the cast iron path = 4X5 = 20 sq. cm. 
'If <£, the total flux = 80,000 lines 
then (Bi, the flux density in the cast steel = 10,000 lines per sq. cm. 
GS 2 , the flux density in the air gaps = 10,000 lines per sq. cm. 
(B 3 , the flux density in the cast iron = 4,000 lines per sq. cm. 
and the ampere turns per cm. for the cast steel = 7, see Fig. 41. 

(B 2 X10 



the ampere turns per cm. for the air gaps 

= 8000 



, see page 33. 



Art. 53] 



SOLENOIDS AND ELECTROMAGNETS 



45 



the ampere turns per cm. for the cast iron = 13, see Fig. 41 

and the total ampere turns = 7 X 20 + 8000 X 2 X 0.05 + 13 X 12 

= 140 + 800 + 156 

= 1096 




the magnetic pull 



Fig. 57. — Annular type of electromagnet. 
(10,000) 2 X 2 X 



dynes 



= 64,000,000 dynes 
= 65,000 gm. 
- 65 kg. 



46 PRINCIPLES OF ELECTRICAL ENGINEERING LChap.viii 

Other values are worked out in the same way, the work generally being 
carried out in tabular form as below : 





Flux density 


Ampere turns per 
centimeter 


Total ampere turns 

















peres 


Pull 


Steel 


Air j Iron 


Steel 


Air 


Iron 


Steel Air 

1 


Iron 


Cir- 
cuit 


64,000 


8,000 


8,000 


3,200 


5 6,400 


9 


100 640 108 


848 


0.848 


42 kg. 


80,000 


10,000 


10,000 


4,000 7 8,000 


13 


140 800 156 


1,096 


1.096 65 kg. 


96,000 


12,000 


12,000 


4,800 11 


9,600 


19 


222 960 228 


1,408 


1.408 94 kg. 


112,000 


14,000 


14,000 


5,600 19 


11,200 


25 


380 1,120 300 


1,800 


1.800 126 kg. 


128,000 16,000 


16,000 


6,400 55 


12,800 


34 


1,100 1,280 408 


2,788 


2.788 164 kg. 



These results are plotted in Fig. 56. 



The possibilities of the annular type of magnet are illustrated 
in Fig. 57. A magnet which weighs 2250 lb. will lift skull cracker 
balls up to 12,000 lb., billets and slabs up to 20,000 lb. and mis- 
cellaneous scrap up to 500 lb. The power required to operate the 
magnet being 11 amp. at 220 volts or 2.42 kw. 

54. Saturation of a Magnetic Circuit. — From the figures in the 
last problem, under the heading of total ampere turns, it may be 
noted that, when the flux densities in the steel and iron are low, 
most of the excitation is required for the air path or most of the 
reluctance of the magnetic circuit is in the air gap. 

When the densities exceed 15,000 lines per sq. cm. for cast steel 
and 6000 for cast iron, the reluctance of the magnetic circuit 
increases rapidly and the curve showing the relation between flux 
and excitation, called the magnetization curve of the circuit, 
bends over rapidly as shown in Fig. 56, the circuit is then said to 
be nearly saturated. 

55. Electromagnetic Brakes and Clutches. — One type of brake 
used on crane motors is shown in Fig. 58. The annular steel 
frame A of the electromagnet is fastened to the housing of the 
motor and carries the exciting coil E. The sliding disc B is 
fastened to the frame A of the magnet by means of a sliding key 
F and is free to move axially but cannot rotate. 

When the motor is disconnected, the magnet is not excited and 
the springs £ push the disc B into the ring C which is keyed to the 
motor shaft, the motor is thereby braked and brought rapidly to 
rest. When current is applied to start the motor, the coil E is 
excited at the same time and the disc B is attracted, releasing the 
ring C, so that the motor shaft is then free to rotate. Electro- 
magnetic clutches are built on the same principle. 



Art. 56] SOLENOIDS AND ELECTROMAGNETS 



47 



56. Magnetic Separator. — A useful application of the electro- 
magnet is shown diagrammatically in Fig. 59. The magnetic 



wynft M JiV 




Fig. 58. — Electromagnetic brake. 



pulley consists of an iron shell containing an exciting coil C which 
produces the magnetic field shown. Any iron particles carried 
over this pulley by the conveyer belt are attracted and are 




Fig. 59. — Magnetic separator. 

therefore carried further round than the nonmagnetic materials 
with which they are mixed. 



CHAPTER IX 

ARMATURE WINDINGS FOR DIRECT -CURRENT 
MACHINERY 

57. Principle of Operation of the Electric Generator. — The 

simplest type of electric generator is shown diagrammatically in 
Fig. 60. If the conductor ab is moved alternately up and down so 
as to cut the lines of force that pass from N to S, an e.m.f . will 
be generated or induced in the conductor which will cause an 
electric current to flow in the closed circuit abed. 

The direction of the current in the conductor ab may be deter- 
mined by the right-hand rule, page 9. The current will reverse 




Fig. 60. — Generation of electromotive force. 

when the direction of motion of the conductor is reversed, so that 
the current will flow first in one direction and then in the other; 
such a current is said to be alternating. 

58. Gramme Ring Winding. — The first satisfactory machine 
that would give a direct current, that is a current which flows con- 
tinuously in one direction, is shown diagrammatically in Fig. 61. 
Since this diagram is rather complicated, the stages in its develop- 
ment will be taken up. 

The poles NS, Fig. 62, are bored out cylindrically and then, in 
order to reduce the reluctance of the magnetic circuit, a soft iron 
core is placed concentrically with the pole faces so as to make_the 
air gap clearances a and b small. In these air gaps the conductors 

48 



Art. 58] 



ARMATURE WINDINGS 



49 



c are moved in the direction of the arrow so as to pass down in 
front of the N pole and up in front of the S pole and thereby cut 
the lines of force that pass from N to S, so that e.m.fs. are gener- 
ated in these conductors, the direction of which, determined by 
the right hand rule, is shown in Fig. 62 at a particular instant. 

The next step in the development of the machine is to attach 
the conductors c to the central core as shown in Fig. 63, so that 




\*F 



/a^B_ 



Fig. 61. — Gramme ring winding. 

they are carried around when the core is driven by a prime mover ; 
the lines of force still pass from N to S and do not rotate with the 
core. When this construction is used it is found necessary to 
laminate the core for the reason given in art. 62, page 55. The 
core area should be large enough to keep the flux density below 
the saturation point, see page 46, so that, while the centre of the 





Fig. 62. Fig. 63. 

Stages in the development of the Gramme ring winding. 

core may generally be cut out as shown so as to save material, 
the depth d must not be made too small. 

It is now necessary to connect the individual conductors to- 
gether so that their voltages add up and this is done by joining 
them as shown in Fig. 64 so as to form an endless helix. The 
complete core and winding form what is called the armature 
of the machine. 



50 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, ix 



Since the lines of force pass through the iron core as shown 
rather than directly across the central air space from m to n, 
only the face conductors c cut these lines, no lines being cut by the 
inner conductors e. The direction of the e.m.f. in each conductor 
is indicated by crosses and dots in the usual way and it may be seen 
that no current can flow through this closed winding because the 
voltages in the conductors under the N pole are opposed by those 
in the conductors under the S pole. A difference of potential 
however will be found between / and g so that if stationary con- 
tacts placed at these two points are connected to an external cir- 
cuit as in Fig. 65, then current will flow through this circuit and 
back through the two paths of the winding as shown. As long 
as the generator rotates in the direction of the arrow, the voltage 





Fig. 64. Fig. 65. 

Stages in the development of the Gramme ring winding. 



distribution will always be as indicated and the voltage between 
the points / and g will be constant in magnitude and direction. 

59. Commutator and Brushes. — Machines have been con- 
structed in which the stationary contacts B- and B + , called the 
brushes, were allowed to rub directly on the winding as shown in 
Fig. 65, but the standard practice is to provide a special rubbing 
contact on each coil such as that shown dotted at s, Fig. 65. The 
complete winding supplied with these contacts is shown diagram- 
matically in Fig. 61 and is also shown in Fig. 66. These rubbing 
contacts form what is called the commutator, and the individual 
contacts are called the commutator segments. 

In Fig. 61, current enters the machine at the negative brush 
B-, passes through the commutator leads a to the winding 



Art. 60] 



ARMATURE WINDINGS 



51 



through which it passes to the positive brush B + and then on to 
the external circuit. The voltage between 2?_ and B + is main- 
tained so long as the armature conductors cut lines of force, while 




Fig. 66. — Armature with a Gramme ring winding. 

the amount of current passing through the machine depends 
entirely on the resistance R of the external circuit. 

60. Multipolar Windings. — It has been found economical in 




Complete Winding 

Fig. 68. 




Diagrammatic Representation of a 4 Pole Winding 

Fig. 69. 
Gramme ring winding for a four pole machine. 

practice to build machines with more than two poles, see page 
56, the poles being arranged in pairs alternately N and S. In 
Fig. 68 the winding for a four -pole machine is shown diagrammat- 



52 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, ix 

ically. The direction of the lines of force and of the e.m.f. in the 
conductors is shown in Fig. 67, from which diagram it may be 
seen that no current can flow in the closed winding because the 
voltages in the conductors under the N poles are opposed by 
equal voltages in the conductors under the S poles. A difference 
of potential however will be found between a and b due to the 
conductors cutting lines of force under pole Si and there is an 
equal difference of potential between a and d due to the conduct- 
ors cutting lines of force under pole iV 2 so that b and d are at the 
same potential and may be connected together. For the same 
reason the stationary contacts a and c may also be connected 
together as shown in Fig. 68. The external circuit to be supplied 
with current is connected between the terminals T + and !T_. 
This current will divide when it enters the machine and pass 
through the four paths in the winding as shown in Fig. 68 and 
also diagrammatically in Fig. 69. 

61. Drum Windings. — One obvious objection to the ring wind- 
ing is that only the ou^er conductors c, Fig. 63, cut lines of force, 




Fig. 70. — Coil of a drum winding. 

the remainder of the winding being inactive. To overcome this 
objection most modern machines have what is called a drum wind- 
ing made with coils which are shaped as shown in Fig. 70 and are 
placed on the surface of the armature core in such a way that the 
conductors a and b are under poles of opposite polarity. The 
e.m.fs. generated in these conductors therefore act in the same 
direction around the coil. 

In Fig. 71, two four-pole machines are shown which are alike 
in every respect except that machine A has a ring coil between 
two adjacent commutator segments whereas machine B has a 
drum coil. 

In Fig. 72, the same two machines are shown with quarter of 
the armature winding in place, the conductors being numbered in 
the order in which they are connected in series. By following 



Art. 61] 



ARMATURE WINDINGS 



53 



each winding from the negative to the positive brush, it will be 
found that in each case the e.m.f. between the brushes is due 





A. Gramme ring coil. B. Drum coil. 

Fig. 71. — Comparison between ring and drum coils. 




A. Gramme ring winding. B. Drum winding. 

Fig. 72. — Comparison between ring and drum windings. 





Fig. 73. — Complete winding. Fig. 74. — Diagrammatic 

representation. 
Drum winding for a four pole machine. 

to the voltage generated in four conductors in series, no voltage 
being generated in conductors 1 and 6 since they are not under 
the poles and so are not cutting lines of force. 



54 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, ix 



Fig. 73 shows the complete drum winding, which has four paths 
between the negative and the positive terminals of the machine 
and may therefore be represented diagrammatically by Fig. 74. 




Fig. 75. — Armature with a drum winding. 

A complete drum winding has the appearance shown in Fig. 75 
but, so far as the generation of voltage is concerned, it produces 
the same result as the ring winding shown in Fig. 66. By choos- 





Fig. 76. — Two turn coil for a drum winding. 

ing a suitable strength of magnetic field, and a suitable number of 
conductors in series between negative and positive brushes, the 
designer is able to wind armatures for different voltages. The 



Art. 62] 



ARMATURE WINDINGS 



55 



coils shown in the diagrams in this chapter have only one turn 
between adjacent commutator segments but, in order that the 
voltages used in practice may be attained, it is generally neces- 
sary to make the coils as shown in Fig. 76 with several turns 
between segments. 

62. Lamination of the Armature Core. — Fig. 77 shows an 
armature core on which may be placed either a ring or a drum 
winding. If this core is made of a solid block of iroL, then, as it 




Fig. 77. — Eddy currents in a solid armature core. 



Fig. 78. — Laminated 
armature core. 



rotates, e.m.fs. are induced in the surface layers and force current 
through the iron in the direction shown, which direction may 
be determined by the right-hand rule. These currents cannot be 
collected and utilized but power is required to maintain them. 

To keep these eddy currents small, a high resistance is placed 
in their path by laminating the core as in Fig. 78, the laminations 
being separated from one another by varnish. 



CHAPTER X 

CONSTRUCTION AND EXCITATION OF DIRECT- 
CURRENT MACHINES 

63. Multipolar Construction. — Fig. 79 shows a two-pole ma- 
chine and also a six-pole machine built for the same output, the 
machines having the same armature diameter and the same total 
number of lines of force crossing the air gaps. The armature core 
of the two-pole machine must be deep enough to carry half of the 
total flux, while in the six-pole machine the total flux divides up 
among six paths so that the core need be only one-third of the 





Two-pole machine. Six-pole machine. 

Fig. 79. — Machines with the same output. 

depth of that of the two-pole machine. For the same reason the 
six-pole machine has the smaller cross section of yoke. 

By the use of the multipolar construction therefore there is a 
considerable saving in material, but this is at the expense of an 
increase in the cost of labor because of the increased number of 
parts to be machined and handled. The number of poles is 
chosen by the designer to give the cheapest machine that will 
operate satisfactorily. 

56 



Art. 64] 



CONSTRUCTION AND EXCITATION 



57 




58 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.x 

64. Armature Construction.— Fig. 80 shows the type of con- 
struction generally adopted for small machines. The armature 
core M is built up of sheet steel laminations which are separated 
from one another by layers of varnish, see page 55. 

The winding shown is of the drum type, see Fig. 75, page 54, 
and the armature coils G are carried in slots F from which they 
are insulated by paper, cotton and mica. It is found that, even 
when embedded in slots, the conductors cut the lines of force 
crossing from pole to pole. 

The core is divided into sections by spacers P, so that air can 
circulate freely through the machine and keep it cool. The core 
laminations and the spacers P areclamped between end heads N 
which carry coil supports L attached by arms shaped like fans. 
The coils are held against these supports by steel band wires W. 

65. Commutator. — The commutator is built of segments J, 
see page 50, which are of hard-drawn copper. These seg- 
ments are separated from one another by mica strips and are then 
clamped between two cones S from which they are separated by 
mica, the segments being thereby insulated from one another 
and from the frame of the machine. The segments are connected 
to the winding through the leads H which, in modern machines, 
have air spaces between one another as shown, so that air is 
drawn across the commutator and between the leads thereby 
keeping the commutator cool. 

66. The brushes, see page 50, are attached to the studs X, 
which studs are insulated from the supporting arm V, and con- 
nection is made from these studs to the external circuit. 

67. Poles and Yoke. — The armature revolves in the magnetic 
field produced by the exciting or field coils A which are wound on 
the poles B. These poles must have sufficient cross section to 
carry the magnetic flux without the flux density becoming too 
high, the same applies to the cast steel yoke C to which the poles 
are attached by screws. 

68. Large generators are similar to small generators such as 
that described above; some changes are generally required in the 
mechanical design because of the heavier parts to be supported 
and also because of the different kinds of service for which ma- 
chines have to be built. In the case of the engine type generator 
shown in Fig. 81 for example, the armature core is built up of 
segments instead of complete rings, while the commutator is 



Art. 69] CONSTRUCTION AND EXCITATION 



59 



supported from the armature spider since the shaft is supplied by 
the engine builder. 



ommutator 




Fig. 81. — Large direct-current generator. 



tfw 



7w? 

b\~<A — 
■ i — > — \ > 





Fig. 82. — Separately excited Fig. 83. — Shunt excited 

machine. machine. 



69. Excitation. — Permanent magnets are used as field poles 
for small machines called magnetos; large machines are supplied 



60 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, x 



with electromagnets the excitation of which can readily be 
controlled. 

When the generator itself supplies this exciting current it is said 
to be self excited; when the exciting current is supplied from 
some external source the machine is said to be separately excited. 
The different connections used are shown in Figs. 82, 83, 84 
and 85. 

Fig. 82 shows a separately excited machine. 

Fig. 83 shows a shunt machine in which the field coils form a 




< j- — > 





Or: 



Fig. 84. — Series excited 
machine. 



Fig. 85. 



-Compound excited 
machine. 



shunt across the armature terminals and have many turns of small 
wire carrying a current I f = E t /Rf, the terminal voltage divided 
by the resistance of the field coil circuit. This exciting current 
seldom exceeds 5 per cent, of the full-load current supplied to the 
external circuit. 

Fig. 84 shows a series machine in which the field coils are in 
series with the armature and have only about 5 per cent, of the 
number of turns that a shunt winding would have, but employ a 
larger size of wire because they have to carry the total current of 
the machine. 



Art. 69] CONSTRUCTION AND EXCITATION 61 

Fig. 85 shows a compound machine in which there are both 
shunt and series field coils. When the shunt coils are connected 
outside of the series coils the machine is said to have a long shunt 
connection; when connected inside of the series coils the connec- 
tion is said to be short shunt. 



CHAPTER XI 
THEORY OF COMMUTATION 

70. Commutation. — As the armature of a direct-current genera- 
tor revolves, the direction of the current in each conductor 
changes while that conductor passes from one pole to that 
adjoining. In Fig. 86 for example, the direction of the current 
in the coil M is shown at three consecutive instants in diagrams 
A, B and C. 

As the armature moves from A to C the brush changes from seg- 




Fig. 86. — Diagram showing the reversal of the current in coil M. 

ment 1 to segment 2 and the current in the coilM is automatically 
reversed. For a short period, as at B, the brush is in contact with 
both segments and, during this interval of time, the coil M is short 
circuited, but no e.m.f. is generated in the coil since it is not 
cutting lines of force, so that no current passes through the short 
circuit. 

When in position B, the coil is said to be in the neutral position 
and the line L is called the neutral line. 

The operation of reversing the current in an armature coil by 

62 



Art. 71] THEORY OF COMMUTATION 63 

means of the brush and commutator segments is called commuta- 
tion. Unfortunately the operation is not so simple as described 
above, because the coils have self induction and resist a change of 
current, and this we shall see causes sparking and gradual deteri- 
oration of the brushes and commutator. It is therefore neces- 
sary to make a detailed study of the subject because of its 
importance. 

To study the variation of the current in the coil being com- 
mutated, the student should draw the brushes B + and B- and 
also the poles N and S, Fig. 87, on a piece of heavy paper, and the 
armature and commutator on tracing paper. The armature 
should then be placed in the magnetic field and the direction 
of the current in a particular coil noted as the armature 
goes through one revolution. Such a model illustrates the 
operation much better than any set of diagrams such as those 
in Figs. 88, 89 and 90. 

71. Theory of Commutation. — Fig. 88 shows part of a machine 
with a ring winding having two turns per coil and with the current 
in the coil M undergoing commutation. The brush B is made of 
copper so that the resistance of the contact between the brush and 
the commutator is negligible. 

In diagram A, the currents I enter the brush through the com- 
mutator lead a. 

In diagram B, the brush makes contact with two segments and 
the current flowing to the brush through the coils under the S 
pole no longer requires to flow round coil M because it has an 
easier path through the lead b, the current in coil M therefore dies 
down to zero because, being in the neutral position, the coil M is 
not cutting lines of force so that no e.m.f. is generated in it to 
maintain the current. 

In diagram D, segment 1 of the commutator is about to break 
contact with the brush, and the coil M carrying no current is 
about to be thrown in series with the coils under the N pole. 
At the instant the contact is broken, as shown in diagram E, the 
current in coil M tries to increase suddenly from zero to a value 
7", but this change of current is opposed by the self induction of 
the coil M, so that the current prefers to pass to the brush across 
the air space x, causing sparking. 

72. Shifting of the Brushes. — For sparkless commutation, it 
is necessary that the current in the coil M shall be reduced to zero 
and then, by some means or other, raised to a value I in the oppo- 



64 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xi 




Fig. 88. Fig. 89. Fig. 90. 

Fig. 88. — With low resistance brushes on neutral line. 
Fig. 89. — With low resistance brushes shifted in the direction of motion. 
Fig. 90.— With high resistance brushes. 

Stages in the process of commutation. 



Art. 73] THEORY OF COMMUTATION 65 

site direction during the time the coil is short circuited by the 
brush, so that when the contact at x is broken, there shall be no 
sudden change of current in the coil. 

This result may be obtained by shifting the brushes forward 
in the direction of motion as shown in Fig. 89 so that, while coil 
M is short circuited, it is in a magnetic field and an e.m.f. is gen- 
erated in it which will produce the required growth of current. 
This magnetic field is called the reversing field. The correspond- 
ing diagrams in Figs. 88 and 89, and particularly diagrams D 
and E, should be carefully compared. 

If the current taken from the generator is increased, the 
strength of the reversing field must also be increased if commuta- 
tion is to be sparkless, so that the brushes must be moved nearer 
to the pole tips and further from the no-load position. The 
brush position must therefore be changed with change of load. 

73. Interpole Machines. — It has been shown in the last para- 
graph that the commutation of a generator is improved if the 




Fig. 91. Fig. 92. 

Diagrams illustrating the principle of the interpole generator. 

brushes are shifted forward in the direction of motion of the 
machine so that the short circuited coils are in a reversing mag- 
netic field, thus in Fig. 91 the brush B + is moved so as to -come 
under the tip of the N pole and the brush B_ is moved so as to 
come under the tip of the S pole. The same result may be accom- 
plished by leaving the brushes in the neutral position and 
bringing an auxiliary n pole over the brush B + and an auxiliary s 
pole over the brush £_, as shown in Fig. 92. These auxiliary 
poles are called interpoles and a machine so equipped is called an 
interpole machine. 

It is desirable that the strength of the reversing field increase 
with the current drawn from the armature and to obtain this 



66 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xi 

result the interpoles are supplied with series field coils as shown 
in Fig. 92. 

74. Carbon brushes have a contact resistance which is gener- 
ally about ten times that of copper brushes. The effect of this 
high contact resistance is to improve commutation, as may be 
seen by a comparison between Figs. 88 and 90. 

In diagram B, Fig. 90, the brush makes contact with segment 2 
and some of the current I that was flowing round coil M now 
flows directly to the brush through lead b. Since however the 
contact with segment 2 is small in area, the current % flowing 
through this contact is small and some of the current I continues 
to flow around coil M. 

As the armature rotates and the contact area between the 
brush and segment 2 increases, the current i h increases and that 
in coil M decreases until, at the instant shown in diagram C, 
the current in this coil has become zero. 

In diagram D, the contact area between the brush and segment 1 
is small while that between segment 2 and the brush is large so 
that the current in lead a is throttled by the high resistance of the 
small contact area and current is forced around coil M in the 
direction shown. As the contact area between the brush and 
segment 1 decreases, the current i a decreases and that in coil M 
increases until, at the instant shown in diagram E, when the 
contact at x is broken, the current in coil M has been raised to the 
value I by this slide valve action of the high resistance brush. 
The contact can then be broken without causing any sudden 
change of current in the coil M and therefore without sparking. 

In the above theory, the action at the positive brush has been 
considered; the action at the negative brush is similar and need 
not be considered separately. The theory applies to a drum 
winding as well as to a ring winding, the only difference between 
the two cases being in the shape of the coil, see Fig. 71. 

By the use of carbon brushes it is possible to operate generators 
from no-load to full-load without shifting of the brushes during 
operation, and for that reason carbon brushes have superseded 
copper brushes on modern machines. 



4t 



% 







CHAPTER XII 
ARMATURE REACTION 

75. The Cross-magnetizing Effect — In Fig. 93, diagram A 
shows the distribution of the magnetic flux in a two-pole machine 
when the field coils are excited and no current is flowing in the 
armature winding; the flux density is uniform under the pole face 
so that the same number of lines of force cross each square centi- 
meter of the air gap between the pole face and the armature 
surface. 

Diagram B shows the distribution of magnetic flux when the 
armature is carrying current, the brushes being in the neutral 




Flux distribution due 
to the field coils 



Flux distribution due to the 
armature winding 

Fig. 93. — Armature reaction with the brushes in the neutral position. 



Resultant flux 
distribution 



position and the field coils not excited. The current passing 
downward in the conductors under the S pole of the machine and 
up in those which are under the N pole causes the armature to 
become an electromagnet with lines of force which pass through 
the armature in a direction determined by the corkscrew 
law, see page 5, and which return across the pole faces to com- 
plete the circuit. 

Diagram C shows the resultant distribution of magnetic 
flux when, as under load conditions, the armature is carry- 
ing current and the field coils are excited; C is obtained by 
combining the magnetic fields of A and B. Under pole tips a and 
c the magnetic field due to the current in the armature is opposite 

67 



68 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xii 



in direction to that due to the current in the field coils while under 
tips b and d the two magnetic fields are in the same direction. 

Since the armature magnetic field is at right angles to that pro- 
duced by the field magnets, the effect produced is called the cross- 
magnetizing effect of armature reaction. 

76. The Demagnetizing Effect. — In a direct -current generator 
the brushes are shifted from the no-load neutral in the direction of 
motion so as to improve commutation, see page 63, the distribu- 
tion of the magnetic flux when the armature is carrying current 
and the field coils are not excited will then be as shown in diagram 
A, Fig. 94. The armature field is no longer at right angles to that 
produced by the field magnets but acts in the direction oz, it 
may however be considered as the resultant of two magnetic fields, 
one in the direction oy, called the cross-magnetizing component 
and the other in the direction ox, called the demagnetizing com- 
ponent because it is directly opposed to the field produced by the 
field magnets. Diagram B, Fig. 94, shows the armature divided 




A B 

Fig. 94. — Flux distribution due to the armature winding when the brushes 
are shifted in the direction of motion. 



so as to produce these two components; the belts of conductors ab 
and cd, when carrying current, tend to demagnetize the machine, 
while the belts ad and be are cross-magnetizing in effect. 

77. Effect of Armature Reaction on Commutation. — When 
interpoles are not supplied, the brushes are shifted forward in the 
direction of motion so that commutation takes place in a reversing 
magnetic field under pole tips a and c, Fig. 93. But it may be 
seen from diagram C, Fig. 93, that the effect of armature reaction 
is to weaken the magnetic field under these pole tips and so impair 
the commutation. 

This effect must be minimized by making the air gap clear- 
ances 8 as large as possible so that there is a large reluctance in the 
path of the cross field. Increasing the air gap also increases 



Art. 77] ARMATURE REACTION 69 

the reluctance of the main magnetic path and, in order to pro- 
duce the required main flux, it is then necessary to increase 
the number of exciting ampere turns on the poles. The machine 
is then said to have a stiff magnetic field because it is not greatly 
affected by armature reaction. 



CHAPTER XIII 
CHARACTERISTICS OF DIRECT -CURRENT GENERATORS 

78. Magnetization or No-load Saturation Curve. — The voltage 
generated in the armature of a direct-current machine, being 
proportional to the rate of cutting lines of force, is proportional to 
the speed and to the flux per pole or 

E = a const. X 4> X r.p.m. for a given machine. 

The flux per pole, and therefore the voltage, increase with the 
excitation, and the curve showing the relation between no-load 
voltage and excitation, the speed being constant, is called the mag- 
netization or the no-load saturation curve of the machine. Such 




O a Exciting Current If 

Fig. 95. — Magnetization curve of a direct current generator. 

a curve is shown in Fig. 95. With no excitation there is a voltage 
e r due to residual magnetism; as the exciting current is increased, 
the flux per pole and the voltage increase in the same ratio until, 
with an exciting current of oa amperes the magnetic circuit begins 
to saturate and the voltage to increase more slowly. 

To obtain such a curve experimentally, the generator is driven 
at a constant speed with no connected load. The field coils 
are separately excited as shown in Fig. 95 and the exciting cur- 
rent is increased by gradually cutting out the resistance r. 

70 



Art. 80] 



DIRECT-CURRENT GENERATORS 



71 



Simultaneous readings of the voltage E and the current I f are 
taken and the results plotted as shown. 

79. Self excitation is made possible by virtue of the residual 
magnetism in the magnetic circuit of the machine. If for example 
a shunt generator, connected as in Fig. 96, is rotating, a small vol- 
tage e r is generated in the armature even with no exciting current 
in the field coils, because the lines of force of residual magnetism 
are being cut. This small voltage sends a small current through 
the field coils, which increases the magnetic flux and causes the 
generated voltage to increase, and this in turn further increases 
the excitation, and so the voltage of the machine builds up. 

The voltage and the exciting current cannot build up indefi- 
nitely because, as the exciting current increases, the magnetic 




Exciting Current If 

Fig. 96 — Magnetization curve of a shunt generator. 



circuit becomes more nearly saturated and the voltage increases 
by a smaller and smaller amount until finally, when the point A is 
reached at which E /If = Rf, the voltage and exciting current 
can increase no further. 

It frequently happens that, when a generator is started up for 
the first time, the e.m.f. generated in the armature due to residual 
magnetism sends a current through the field coils in such a direc- 
tion as to oppose the residual flux, and the voltage, instead of 
building up, is reduced to zero. In such a case it is necessary 
to reverse the connections of the field coils so as to pass current 
through them in the opposite direction. 

80. Regulation Curve of a Separately Excited or of a Magneto 
Generator. — This curve, sometimes called the external character- 



72 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xiii 

istic, gives the relation between E t , the terminal voltage, and I a , 
the line current, and is shown in Fig. 97 for the case of a separately 
excited machine operating at constant speed and with constant 
excitation. The terminal voltage drops as the current taken from 
the machine is increased because: 

a. The flux per pole is reduced by armature reaction, see page 
68, so that Eg, the e.m.f. generated by cutting this flux, is also 
reduced. 

b. The terminal voltage E t is less than the generated voltage 
Eg by the armature resistance drop I a R a , that part of the gener- 
ated voltage required to force the armature current through the 
resistance of the armature winding and of the brush contacts. 



Drop Due to Armature Reaction 
Armature Resistance Drop Ia^a 





Line Current I a 

Fig. 97.— Regulation curve of a separately excited generator. 



To obtain such a curve experimentally, the generator is loaded 
on a bank of lamps ; or some other suitable load that can readily be 
adjusted, as shown in Fig. 97. The speed and the exciting cur- 
rent If are kept constant, while the current taken from the 
machine is gradually increased by connecting an increasing num- 
ber of lamps in parallel across the terminals, that is by providing 
more paths through which current can pass. Simultaneous 
readings of the voltage E t and of the current I a are taken and the 
results plotted as in Fig. 97. 

The regulation of the above generator is defined as the per cent, 
change in voltage when full-load is thrown off the machine, the 
speed and the field circuit being unchanged. The regulation 
therefore = (E - E t )/E t . 

81. Regulation Curve of a Shunt Generator. — This curve is 
shown in Fig. 98 for a constant speed shunt excited generator. 



Art. 81] 



DIRECT-CURRENT GENERATORS 



73 



The terminal voltage drops as the current taken from the machine 
is increased because: 

a. The flux per pole is reduced by armature reaction. 

b. The armature drop I a R a is used up in the machine itself. 

c. The exciting current I f is equal to E t /Rf, where R f is the 
constant resistance of the shunt field circuit, so that as the ter- 
minal voltage drops the exciting current decreases and causes the 
voltage to drop still further. Because of this third effect the 
terminal voltage of a generator with a given load will be lower 
when the machine is shunt excited than when separately excited. 



VDrop Due to Armature 

tr — Reaction 

4^ Armature Resistance 
Drop I a R a 
Drop Due to Decrease 
in Excitation 

If I] External 

- J — Circuit 





Line Current i"j ly^i 

Fig. 98. — Regulation curve of a shunt generator. 



To obtain such a curve experimentally the machine is connected 
up as shown. The speed and the resistance of the shunt circuit 
are kept constant while the current taken from the machine is 
gradually increased, and simultaneous readings are taken of the 
voltage E t and the current I h these results are plotted as in 
Fig. 98. 

As the resistance of the external circuit is decreased, the current 
supplied by the machine increases and the terminal voltage 
drops until point d is reached. A further reduction in the external 
resistance allows an increased cunent to flow for an instant, but 
this increase of current reacts by armature reaction and causes 
such a large drop in voltage and in exciting current that the arma- 
ture current cannot be maintained. In the extreme case when 
the generator is short circuited, that is, the terminals of the ma- 
chine are connected through a circuit of negligible resistance, then 
the terminal voltage must be zero and there can be no field excita- 



74 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xiii 

tion, so that no current can flow in the short circuit; thus c, 
Fig. 98, is a point on the load curve. 

If a shunt generator is short circuited, it carries the maximum 
current I m for only a short interval of time before it loses its 
voltage and is thereby protected from injury. For the same 
reason a shunt generator will not build up if a very low resistance 
is connected across its terminals. Because of this self protecting 
power, shunt generators are used with advantage for electric fur- 
nace and other work during the process of which the machine is 
liable to be short circuited. 

82. To maintain the terminal voltage constant, shunt gen- 
erators are operated with an adjustable resistance, called a field 



Over Compound 
Generator 




Line Current Ii 

Fig. 99. — Regulation curves of compound generators. 

rheostat, placed in the field coil circuit. As the load on the 
machine increases and the voltage drops, some of this resistance 
may be cut out either automatically or by hand so as to increase 
the excitation. Automatic regulators for this purpose are used 
with alternating-current generators, see page 249, but are sel- 
dom used with direct-current generators because the same result 
may be obtained more cheaply by the use of compound windings. 
83. Compound generators, operated without a regulator, 
maintain the terminal voltage approximately constant from no- 
load to full-load, because the line current passes through the series 
field coils and causes the total excitation to increase with the 
load. By the use of a large number of series turns, the total 
excitation may increase so much with the load that the terminal 
voltage will rise, as shown in curve B Fig. 99, the machine is then 



Art. 84] 



DIRECT-CURRENT GENERATORS 



75 



said to be overcompounded. When the terminal voltage has the 
same value at full-load as at no-load, the machine is said to be flat 
compounded. 

Generators for lighting and power service are generally flat- 
compound machines wound for 125 or for 250 volts. For railway 
service the generators are overcompounded so as to maintain the 
trolley voltage at some distance from the power house. Street 
railway generators are invariably wound for 600 volts at full- 
load, while for interurban and trunk line work 2400 volts has been 
used. 

84. The Regulation Curve of a Series Generator. — Curve A, 
Fig. 100, shows what the relation between voltage and current 




Line Current la 

Fig. 100. — Regulation curve of a series generator. 

in a series generator would be if armature resistance and armature 
reaction were negligible; the voltage would increase with the load 
current since this is also the exciting current. Curve A is really 
the no-load saturation curve of the machine and is determined by 
separately exciting the field coils, as shown in diagram A, so that 
no current flows in the armature. Curve B shows the actual rela- 
tion between terminal voltage and load current; the drop of 
voltage between curves A and B consists of the portion due to the 
reduction in the flux per pole caused by armature reaction, and 
I a R a the drop of voltage in the armature winding, brush contacts 
and series field coils. 

Series generators were formerly used as constant-current gen- 
erators for the operation of arc lamps in series. They were 
operated with automatic regulators so as to have the line ab, Fig. 
100, nearly vertical, and the current practically constant for all 
voltages up to E m . 



76 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xii. 

A very simple type of regulator for this purpose is shown dia- 
grammatically in Fig. 101, where C is a carbon pile iheostat and 
B is a solenoid carrying the line current. If the current in the 
external circuit increases, the pull of the solenoid B also increases 
and the carbon pile is compressed and its resistance thereby 
decreased, see page 29, so that it shunts more of the current from 
the series field coils. The flux in the machine is therefore reduced, 
and the voltage drops until the current in the line reaches the 
value for which the pull of the solenoid was adjusted. 



K>w 




*-*- 



Fig. 101. — Automatic regulator for a constant-current generator. 



85. Problem on Generator Characteristics.— a. A direct-current shunt 
generator was tested with the brushes shifted forward in the direction of 
motion. The voltage drop between no-load and full-load was 6 volts, what 
are the causes of this drop in voltage? 

b. If the brushes had been placed on the neutral position, why would the 
voltage drop have been different and what would you expect its value to be. 
Why are the brushes not placed on the neutral position in non-interpole 
machines? 

c. Series field coils were added to the machine and the voltage dropped 20 
volts from no-load to full-load, what was the cause of this excessive drop? 

d. After the series field coil circuit has been fixed, the voltage was found 
to increase by 6 volts from no-load to full-load while flat compounding was 
desired, what changes would you suggest should now be made? 

a. The voltage drop is due to: 

1. The reduction in the flux per pole due to the demagnetizing effect of 
armature reaction, since the brushes are shifted forward. 

2. The armature resistance drop. 

3. The reduction in the exciting current which causes the flux per pole to 
decrease and the voltage to drop still further, see page 73. 

6. When the brushes are placed in the neutral position, the armature reac- 
tion has no demagnetizing effect, see page 68, so that the voltage drop will be 
less than when the brushes are shifted forward, and will probably not exceed 
4 volts. 



Art. 85] 



DIRECT-CURRENT GENERATORS 



77 



Series Shunt 



Machines have the brushes shifted forward in order to improve com- 
mutation and, unless interpoles are supplied, these machines will generally 
spark at the commutator on full-load if the brushes are kept on the no-load 
neutral. 

c. Since the voltage drop when the series 
field was added was greater than before, 
it is evident that this field has been con- 
nected backward so as to oppose the 
shunt field instead of assist it, the series 
connections must therefore be reversed so 
that the current passes through the series 
coils in the proper direction. 

d. Since the compounding effect of the 
series coils is too large, it will be necessary 
to reduce the number of series turns or to 
reduce the current flowing through these 

turns. The latter method is that generally adopted, a shunt being placed 
in parallel with the series coils, as shown in Fig. 102, so that, of the total 
current Ii, only a fixed portion passes through the series field coils. 




Fig. 102. — Series shunt to 
vary the series excitation. 



CHAPTER XIV 
THEORY OF OPERATION OF DIRECT -CURRENT MOTORS 

86. Driving Force of a Motor. — An electric generator and an 
electric motor are identical in structure. The geneiator is used 
to transform mechanical energy into electrical energy, while the 
same machine operating as a motor can be used to transform elec- 
trical energy into mechanical energy. 

If a voltage is applied at the terminals of the machine in 
diagram B, Fig. 103, so as to send current through the armature 
conductors in the direction shown, then, since these conductors 
are carrying current and are in a magnetic field, they are acted on 
by forces all of -which act in the same direction around the shaft 
and so cause the armature to rotate. 

87. Driving and Retarding Forces in Generators and Motors. — 
The generator in diagram A, Fig. 103, driven by an engine in the 



■Retarding Force 



p T jv ing F ore; 





Fig. 103. — Driving and retarding forces in a generator and in a motor. 

direction shown, supplies electric power to a circuit, and current 
flows through the armature conductors in the direction indicated 
by the crosses and dots. There is a force exerted on these conduc- 
tors in as much as they are carrying current in a magnetic field, 
which force is opposed to the direction of motion, see page 13, 
and the larger the current the greater is this retarding force. To 
keep the generator running, the driving force of the engine must 

78 



Art. 88] OPERATION OF DIRECT-CURRENT MOTORS 79 

be great enough to overcome this retarding force and also to over- 
come the friction force of the machine. 

The same machine operating as a motor is shown in diagram B. 
A voltage applied at the motor terminals from some external 
source forces current through the armature conductors in the di- 
rection shown and, since these conductors are carrying current in a 
magnetic field, they are acted on by forces which cause the arma- 
ture to rotate in a direction that may be determined by the left- 
hand rule, page 7. Now the conductors, rotating with the 
armature, cut lines of force, and an e.m.f. is generated in the wind- 
ing in exactjy the same way as if the machine was driven by an 
engine. This e.m.f. acts in the same direction as in diagram A 
since the machines have the same polarity and rotate in the same 
direction. This generated e.m.f. is therefore opposed to the 
current in the conductors and opposed to the applied e.m.f., 
for which reason it is called the back or counter e.m.f. of the 
motor. 

In the case of both a generator and a motor, there is a force 
acting on the conductors of the armature in as much as they are 
carrying current and are in a magnetic field. This is the driving 
force in the case of a motor and the retarding force in the case of a 
generator. There is also an e.m.f. generated in the armature of 
each machine in as much as it is rotating in a magnetic field. 
This e.m.f. acts in the direction of the current flow in the case of 
a generator but opposes the current flow in the case of a motor. 

In order that current may flow through a motor armature, the 
applied e.m.f. E a must be greater than the back e.m.f. E b and 

E a = Eb + I a R a 

where E a is the applied voltage 
E b is the back e.m.f. 

I a R a is the voltage required to force the armature current 
I a through the armature resistance R a and is called the arma- 
ture resistance drop. 

In the above equation it is most important to note that, of the 
applied voltage E a , the part which forces the current through the 
resistance of the armature is I a R a and seldom exceeds 5 per cent, 
of E a ', the remaining part of the applied voltage is required to 
overcome E b , the back generated voltage of the machine. 

88. The Back E.M.F. — The existence of the back e.m.f. may 
readily be shown by experiment. If for example a motor, con- 



80 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xiv 



nected as shown in Fig. 104, is driving a flywheel, and the switch 
S is suddenly opened so as to disconnect the armature from the 
power mains, the flywheel will keep the machine running, but the 
ammeter reading will become zero and the voltmeter reading will 
drop suddenly from E a to E b , the voltage generated by the rotat- 
ing armature, and will then drop slowly to zero as the motor comes 
to rest. 




Fig. 104. — Experimental determination of the back e.m.f. 



Example: Let E a , the applied voltage, be 110, I a the armature current, 
100 amp., and R a the armature resistance, 0.04 ohms. The voltmeter 
reading will be 110 volts while switch S is closed but will drop suddenly to 
110 — (100 X 0.04) = 106 volts at the instant the 1 switch is opened, and will 
then drop slowly to zero as the motor slows down. 

89. Theory of Motor Operation. — The power taken by a motor 
from the mains changes automatically to suit the mechanical 
load. Consider the case of a motor connected as shown in Fig. 
105, the applied voltage, the exciting current If, and the magnetic 
flux per pole being constant. If the motor is at standstill and 
the switch S is closed, a large current I a = E a /R a will flow 
through the armature, the back voltage Eb being zero since the 
armature conductors are not cutting lines of force. The arma- 
ture conductors carrying current, being in a magnetic field, are 
acted on by forces which overcome the resisting forces of friction 
and of the load and cause the motor to rotate. As the motor 
increases in speed, the back e.m.f. Eb also increases since it is pro- 
portional to the rate at which the armature conductors cut lines of 
force, and therefore the current I a = (E a — E b )/R a , see page 
79, decreases. The motor will stop accelerating when this cur- 
rent has dropped to such a value that the total force developed is 
just sufficient to overcome the retarding force. 

If now the load on the motor is increased, the driving force due 
to the armature current is not sufficient to overcome the increased 
resisting force and the motor must slow down. As the speed 



Art. 89.] OPERATION OF DIRECT-CURRENT MOTORS 



81 



decreases, however, the back e.m.f. E b also decreases and allows a 
larger current to flow through the armature, since I a = (E a — 
E b )/R a . The motor finally settles down to such a speed that the 
increased current in the armature again produces a driving force 
which is just sufficient to overcome the increased retarding force. 
If the load on the motor is decreased, the driving force due to 
the armature current is more than sufficient to overcome the 
decreased resisting force and the motor must accelerate. As it 
increases in speed, however, the back e.m.f. E b also increases and 
causes the armature current I a to decrease. The motor stops 
accelerating and the speed and 
armature current remain con- 
stant when the driving force 
due to the current has dropped 
to such a value that it is just 
sufficient to overcome the de- 
creased retarding force. The 
electrical power taken by the 
motor from the mains there- 
fore changes automatically to 
suit the mechanical load on the 
motor. The back e. m.f . of the 

motor regulates the flow of current in the same way as the gover- 
nor regulates the flow of steam in a steam engine. 

A 110-volt direct- current motor, connected to the mains as shown in Fig. 105 
delivers 10 h.p. If the efficiency is 88 per cent., the exciting current 
is 2 amp. and the armature resistance is 0.08 ohms find: 

a. The motor input 

b. The current taken from the mains 

c. The armature current 

d. The back e.m.f. 




Fig. 105. 



a. the motor output 
the motor input 



10 h.p. 
output 



efficiency 

10 
0.88 
11.35 X 746 



= 11.35 h.p. 

8480 watts 



b. h, the current from the mains 



watts input 

applied voltage 

8480 mm 

= 77 amp. 

110 



82 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xiv 

c. the armature current = the total current — the exciting current 

= 77 - 2 = 75 amp. 

d. the applied voltage =110 
the voltage to overcome the resistance drop = I a R a 

= 75 X 0.08 
= 6 volts 
the back e.m.f. Eb = E a — I a R a 
= 110 - 6 
= 104 volts. 

90. Speed and Torque Formulae. — The force on a conductor 
carrying current in a magnetic field is proportional to the current 
and to the strength of the magnetic field, see page 7, so that the 
torque developed by a given motor may be expressed by the 
equation 

T = a const. X <f> X I a 

where T is the torque in lb. at 1 ft. radius, or in kg. at lm. radius 
is the flux per pole of the motor 
I a is the armature current 

the constant depends on the construction of the machine and 
on the units chosen, but with its actual value we are not 
concerned. 

When a motor is running, the back e.m.f. is always less than 
the applied e.m.f. by I a R a the armature resistance drop, see page 
79, so that 

Eb = E a — I a R a 

Now Eb is generated in the motor armature because the con- 
ductors are cutting lines of force, see page 79, so that, in a given 
machine, Eb is proportional to the flux per pole and to the speed 
Or 

E b = k<j> r.p.m. where k is a constant 
and E b = E a — I a R a as shown above 

therefore r.p.m. = t— 

= a const. X 

where r.p.m. is the motor speed in revolutions per minute 
E a is the voltage applied at the motor terminals 
I a R a is the armature resistance drop. 
</> is the flux per pole of the motor. 



Art. 92] 



OPERATION OF DIRECT-CURRENT MOTORS 



83 



These speed and torque formulae will be used in the next 
chapter for the determination of the characteristics of different 
types of motors. 

91. Improvement of Commutation by Shifting of the Brushes. — 
In a direct-current generator the brushes are shifted from the 
neutral in the direction of motion so that the coil in which the 
current is being reversed is in what has been called a reversing 
field, see page 65; in the case of the generator shown in diagram 
A Fig. 106, this reversing field is under the tip of the north pole. 

The same machine operating as a motor is shown in diagram B, 
the direction of motion and the polarity of the poles being- 
unchanged, while the direction of the current is reversed in order 
to make the motor rotate in the desired direction. If then the 
reversing field for the conductor at brush B of the generator 




A B 

Generator Motor 

Fig. 106. — Armature magnetic field in a generator and in a motor. 



is under tip of the N pole, that for the conductor at the same brush 
of the motor must be under the tip of the S pole since the motor is 
running in the same direction as the generator but with a reversed 
current. From this result the rule is obtained that in a generator 
the brushes should be shifted forward in the direction of motion 
whereas in a motor they should be shifted backward against the 
direction of motion. 

92. Armature Reaction in Generators and Motors. — In Fig. 
106, which shows a generator and a motor respectively with the 
brushes shifted so as to improve commutation, the distribution of 
magnetic flux due to the armature acting alone is as shown by the 
lines of force. The armature field acts in the direction oz and 
may be considered as the resultant of a cross magnetizing com- 
ponent in the direction oy and of a demagnetizing component in 



84 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xiv 

the direction ox, see page 68, and, in the case of both the genera- 
tor and the motor, the most important effects of the reaction of 
the armature field on that due to the exciting current in the field 
coils are, that the demagnetizing effect reduces the flux per pole, 
while the cross-magnetizing effect causes the flux density to 
decrease under the pole tips toward which the brushes have been 
shifted, a condition which tends to cause poor commutation, 
see page 68. 



CHAPTER XV 
CHARACTERISTICS OF DIRECT -CURRENT MOTORS 

SHUNT-WOUND MOTORS 

93. The Starting Torque. — The shunt motor is connected 
to the power mains as shown diagrammatically in Fig. 107. 
The applied voltage E a and the exciting current I f are constant 
and are independent of the armature current I a . 

To start such a machine, the field coils are fully excited so 
that the magnetic flux has its normal value and then the re- 
sistance R s is gradually decreased and current allowed to flow 
through the armature. The torque developed increases directly 
as the armature current is increased and the motor will start to 
rotate when the current has such a value that the torque devel- 
oped is large enough to overcome the resisting torque of friction 
and of the load. 

The torque developed, being equal to k<f>I a , see page 82, 
depends only on the flux per pole and on the armature current 
and, since the exciting current and therefore the flux per pole are 
constant, full -load current in the machine produces the same 
torque at starting as when the motor is running at full-load and 
normal speed, or full-load torque is developed with full-load 
current; similarly twice full-load torque is developed with twice 
full-load current in the armature. 

94. The Starting Resistance. — If a motor armature at stand- 
still were connected directly to the power mains then, since its 
resistance is small, a large current would flow through the armature 
and burn the windings and the brushes. To limit the starting 
current, a starting resistance must be inserted in series with the 
armature as shown in Fig. 107 and, if full-load torque is required 
at starting, this resistance must limit the current to its normal 
full-load value. 

As soon as the armature begins to rotate, a back e.m.f. is 

generated in it which tends to make the current decrease since 

( TP 7? ^ 
I a = / n" v p \ but, to maintain full-load torque until the 

\£l a ~T~ lis) 

85 



86 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xv 



motor is up to speed, full-load current must be maintained in 
the armature, so that the starting resistance must gradually be 
decreased as the motor comes up to speed and the back e.m.f. 
increases. 

A 10-h.p., 110 volt, direct-current shunt motor has an efficiency of 88 
per cent., an exciting current of 2 amp. and an armature resistance of 0.08 
ohms find: 

a. The starting resistance required for full-load torque 

b. The starting current if no starting resistance was used 
output 



a. the motor input 



efficiency 
10 



0. 



11.35 hrp. 



= 11.35 X 746 = 8480 watts 
the current from the mains 



watts input 
applied voltage 
8480 w 

110 = amp ' 




Fig. 107. — Connec- 
tions of a shunt motor 
during starting. 




Fig. 108.— Starter for 
a shunt motor. 



Fig. 109.— Starter 
with a no-voltage re- 
lease. 



the armature current = the total current — the exciting current 
= 77 - 2 = 75 amp. 

applied voltage 



the total resistance at starting 



full-load armature current 

HO , ,- , 
-=k" = 1.47 ohms. 



the starting resistance = the total resistance — the armature resistance 
= 1.47 - 0.08 
- 1.39 ohms 



Art. 95] DIRECT-CURRENT MOTORS 87 

b. the starting current if no starting resistance is used 

applied voltage 
armature resistance 

= QQg = 1380 amp. 

= 18.4 times full-load current, which would burn up 
the winding. 

95. Motor Starter. — A starter which may be used to perform 
the operations described above is shown diagrammatically in 
Fig. 108. When the handle A, which is made of metal, is moved 
into position Ai, the field coils are fully excited while the arma- 
ture and the whole starting resistance are put in series across the 
power mains. As the handle is gradually moved over to position 
A 2, the starting resistance is gradually cut out of the armature 
circuit, but the current 1 f in the field coils remains practically 
unchanged since the starting resistance R s is small compared 
with R f , the resistance of the field coils; in the above problem, for 

example, R s = 1.39 ohms while R f = -j- = -~- = 55 ohms. 

The starting handle must not be moved over too rapidly, or the 
starting resistance will be cut out before the speed and therefore 
the back e.m.f. have time to increase and limit the current. The 
handle however must not be left on one of the intermediate 
notches between A\ and A^ because, in order to keep down the 
cost of the starting resistance, it is made small and will not carry 
full-load current without injurious heating for more than about 
15 sec. 

96. No-voltage Release. — Suppose that a motor is running at 
normal speed and that the power supply is interrupted due to some 
trouble in the power house or in the line, the motor will stop, but 
the starting handle will remain in the running position. If 
the power supply is now re-established, the armature will be at 
standstill and there will be no starting resistance in series with it 
to limit the current. To take care of such a contingency the 
starter is changed by the addition of what is called the no-voltage 
release. A starter with this attachment is shown diagrammat- 
ically in Fig. 109. The starting handle is moved from the starting 
to the running position against the tension of the spring S and is 
held in the running position by the electromagnet M. If the 
power supply is now interrupted, the exciting current will 
decrease, the magnet M will not be able to hold the handle 



88 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xv 



against the pull of the spring, and the handle will be pulled back 
to the starting position. The magnet M is generally so designed 
that it will release the starting handle should the applied voltage 
drop below 30 per cent, of its normal value. Such starters are 
described more fully in Chapter 19, page 114. 

97. Load Characteristics. — The characteristic curves of a motor 
show how the torque and the speed vary with the armature 
current, the applied voltage being constant. These curves may 
readily be determined from the formulae: 





Torque = k 4>Ia 

. (Ea-IaBa) 



£7 a and0are Constai t 



Armature Current I a 

Fig. 110. — Characteristic curves of a shunt motor. 



torque = k(f>I a 

(E a — I a Ra) 

r.p.m. =ki 

where E a is the applied voltage 

I a is the armature current in amperes 
R a is the armature resistance in ohms 
I a R a , the armature resistance drop, seldom exceeds 5 per 
cent, of E a when the motor is carrying full-load 
4> is the flux per pole 
k and k\ are constants 

In the case of the shunt motor, see Fig. 110, the applied voltage 
E a and the exciting current I f are constant and so also is the flux 
per pole, the effect of armature reaction being neglected, then: 

torque = k<f>I a 

= a const. X I a 

(E a — IaRa) 

r.p.m. 



= ky 

= a const. 





(E a 



IaRa) 



Art. 98] 



DIRECT-CURRENT MOTORS 



89 



The curves corresponding to these equations are shown in 
Fig. 110. The full-load speed is less than that at no-load by 
about 5 per cent, since the back e.m.f. E b has to drop this 
amount in order that full-load current may flow through the 
armature. 

98. Effect of Armature Reaction on the Speed. — When the 
effect of armature reaction is neglected, the speed characteristic 
of a shunt motor is as shown in Fig. 110; the drop in speed seldom 
exceeds 5 per cent, at full-load. When the brushes are shifted 
backward from the neutral so as to improve commutation, arma- 
ture reaction causes the flux per pole to decrease as the load in- 
creases, see page 84, so that the speed, being equal to k(E a — 
I a Ra)/(f> remains approximately constant from no-load to full- 
load, since the decrease in the value of (E a — I a R a ) is compen- 
sated for by the decrease in the value of <£. 

Shunt motors are suited for constant-speed work such as the 
driving of line shafts and wood -working machinery. 

99. Variable speed operation can best be investigated by 
means of the equation r.p.m. = k(E a — I a R a )/4>, see page 82. 




Fig. 111. — Insert resis- 
tance in the field circuit to 
increase the speed. 



Fig. 112. — Insert resistanc^n the 
armature circuit to decrease the 
speed. 



Methods of adjusting the speed of a shunt motor. 



To increase the speed, the flux per pole must be reduced by in- 
serting a resistance in series with the field coils as in Fig. 111. 
To decrease the speed below the value which it has when the flux 
per pole is a maximum, the voltage E a applied to the motor 
terminals must be decreased by inserting a resistance in 
series with the armature as shown in Fig. 112; this resistance must 
be able to carry the full-load current without inj ury so that the 
starting resistance must not be used since it is designed for start- 
ing duty only, see page 87. 

While the formula shows that the speed increases when the 



90 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xv 

flux </> is decreased, it is advisable to study more fully how this 
takes place. If the flux per pole is suddenly decreased, the back 
e.m.f. of the motor drops and allows more current to flow in the 
armature. The increase in the armature current is greater than 
the decrease in the flux, so that the torque developed is greater 
than necessary for the load and the motor accelerates. The 
following problem illustrates this. 

A 10-h.p., 110 volt, 900 r.p.m. direct- current shunt motor has an 
armature resistance of 0.08 ohms and takes an armature current of 75 
amp. at full load. Find: 

a. The torque at full -load 

b. The back e.m.f. at full-load 

If the flux per pole is suddenly reduced to 80 per cent, of normal find: 

c. The back e.m.f. at the instant the flux is changed 

d. The armature current at the same instant 

e. The torque at the same instant 

horse-power X 33,000 
a. the torque = — 



2ttt. p,m. 
10 X 33,000 



2tt900 



= 58.5 lb. at 1 ft. radius 



b. the back e.m.f. Eb = E a — I a R a 

= 110 - (75 X 0.08) = 104 volts at full-load 

At the instant the flux is reduced 

c. the back e.m.f. E b = 104 X 80/100 = 83.2 volts, since is reduced 

d. the armature current = {E a — Eb)/R a 

= (110 - 83.2) /0.08 

= 335 amp. or 4.46 times full-load current 

e. the torque = 58.5 X 80/100 X 335/75, since it is proportional to the 

flux and to the armature current 
= 209 lb. at 1 ft. radius or 3.6 times full-load torque. 

/. At the instant the flux per pole is reduced to 80 per cent, of its normal 
value, the armature current increases to 4.46 normal and the torque to 3.6 
times normal value. The driving torque being then larger than the retarding 
torque of the load, the motor must accelerate. 

SERIES-WOUND MOTORS 

100. The Starting Torque. — The series motor is connected to 
the power mains as shown diagrammatically in Fig. 113. The 
applied voltage E a is constant while the field excitation increases 
with the load. 



Art. 101] 



DIRECT-CURRENT MOTORS 



91 



The torque developed, being equal to k<t>I a , see page 82, 
increases directly with <f> the flux per pole and with I a the 
armature current. Now </> increases with I a since that current 
is also the exciting current and, if the magnetic circuit of the 
machine is not saturated, <f> is directly proportional to I a and the 
torque is therefore proportional to 7 2 . In an actual motor, the 
flux per pole does not increase as rapidly as the exciting current, 
due to saturation of the magnetic circuit, but varies with I a 
as shown in curve 1, Fig. 113. Using this relation between 
'<f> and I a , the relation between torque (fc#J a ) and I a has been 
determined and is plotted in curve 2. 

Full-load current in the machine producer the same flux per 




s 








Pi 


Vcj3 




M 


V 




■W 


V 3 




§ 


\3 




o 


V" 




3 


w 


^ 


a> 




y . 


o 


\o 


y / 


H 


V 


y. / 


3 




s / 


i-i 


B.p.m. Shunt/ 


S. | / ,' 




- z -__ 


"n^^//--- 


Torque = k 0/« 




/' 





/] 


Ea^ 5 Constant 
Increases with la 




9T3 

O 






/ X/^* 


9 





Armature Current 2a 

Fig. 113 — Characteristic curves of a series motor. 



pole and therefore the same torque at starting as when the motor 
is running at full-load and normal speed, or full-load torque is 
developed with full-load current. Since the torque is approxi- 
mately proportional to I a 2 , twice full-load torque is developed 
with approximately V2 times or 1.414 times full-load current. 
In the case of the shunt motor, the flux per pole is constant and 
the torque is directly proportional to I a , see page 85, so that 
twice full-load torque requires twice full-load current. For 
heavy starting duty, therefore, the series motor is better than the 
shunt motor in that it takes less starting current from the line. 
101. The Starting Resistance. — As in the case of the shunt 
motor, see page 85, a starting resistance must be inserted 
in series with the armature so as to limit the starting current. 



92 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xv 

This resistance must be gradually decreased as the motor comes 
up to speed. 

102. Load Characteristics. — The characteristic curves of a 
series motor may readily be determined from the fundamental 
f ormulse : 

torque = k<j>I a 

-. (E a — I a R a ) 

r.p.m. = /Ci 



where E a is the applied voltage 

I a is the armature current in amperes 
R a is the combined resistance of the armature and 
the series field coils 
IaRa, the armature and series field drop, seldom ex- 
ceeds 7 per cent, of E a when the motor is carry- 
ing full-load 
is the flux per pole 
k and fci are constants 

In the case of the series motor, the applied voltage E a is 
constant, while the flux per pole varies with I a as shown in curve 
1, Fig. 113. The relation between torque (fc0/ o ) and arma- 
ture current is plotted in curve 2, while curve 3 shows the rela- 

tion between r.p.m. ( fci— ^ — ° ) and the armature current. 

It is important to note that, as the load and therefore the 
armature current decrease, the flux per pole decreases and the 
machine must speed up to give the required back e.m.f . At light 
loads the speed becomes dangerously high and for this reason a 
series motor should always be geared or direct connected to the 
load. If a series motor were belted to the load and the belt broke 
or slipped off, then the motor would run away and would prob- 
ably burst. 

Series motors are suited for crane work because they develop 
a large starting torque, slow down when a heavy weight is being 
lifted and speed up with light loads. Crane motors are geared 
to the hoisting drum and are always under the control of 
the operator. 

103. Speed Adjustment. — The speed of a series motor is 
proportional to (E a — I a R a )/(f), see page 82, so that, for a given 
current I a . the speed may be changed by altering E a the applied 
voltage, or the flux per pole. 



Art. 104] DIRECT-CURRENT MOTORS 93 

If a resistance R e is inserted in series with the armature as 
shown in Fig. 114, then the voltage applied at the motor terminals 
is reduced by I a R e and the lower back e.m.f. required is obtained 
at a lower speed. 

With constant applied voltage and a given amature current 
the speed may be increased by decreasing the flux per pole. This 
may be done as shown in Fig. 115 by shunting the series field 
winding with a resistance so that, of the total current I a , only 
part is allowed to pass through the field winding. The flux per 
pole may also be reduced by short circuiting part of the field wind- 
ing as shown in Fig. 116, if the switch S is closed, the current 




Fig. 114. Fig. 115. Fig. 116. 

Fig. 114 — Insert resistance in the armature circuit to reduce the speed. 
Fig. 115. — Shunt the field coils to reduce the excitation and increase the 
speed. 

Fig. 116— Short circuit part of the field winding to reduce the excitation 
and increase the speed. 

Methods of adjusting the speed of a series motor. 

passing through the machine is not changed so long as the load is 
kept constant, but the exciting ampere turns are reduced and so 
therefore is the flux per pole. 



COMPOUND MOTORS 

104. The compound motor is a compromise between the shunt 
and the series motor and is connected to the power mains as 
shown diagrammatically in Fig. 117. The applied voltage E a is 
constant and so also is the shunt current I f , but the current in 
the series field coils increases with the load, so that the flux per 
pole increases with the load but not so rapidly as in the series 
motor. 

If a shunt and a compound motor have duplicate armatures 
and the same excitation at full-load, then at this load they will 



94 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xv 

develop the same torque and run at the same speed since 

torque = kcj)I a 

(Ea—IaRa) 

r.p.m. = fci 

For loads greater than full-load, the flux per pole of the shunt 
motor is unchanged while that of the compound motor is in- 
creased due to the series field coils, therefore the compound motor 
has the greater torque but the lower speed. For loads less than full- 
load on the other hand, the flux per pole of the compound motor is 
less than that of the shunt motor due to the decrease of the 
current in the series field coils, so that the torque is less and the 
speed is greater than in the shunt machine. 

The speed and torque characteristics of a compound motor 






^ i 






^V * 


/ / 




K.p.m. Sbunt**~>>^i 


/ / 


a 


// 




» 






M 












a 


</\ 






JWA 




H 








Armature Current 


la 



Fig. 117. — Characteristic curves of compound motors. 

are shown in Fig. 117. Unlike the series motor, the compound 
motor has a safe maximum speed at no-load and so cannot run 
away on light loads. The speed of a compound motor may be 
decreased below normal by means of a resistance inserted in the 
armature circuit, and increased above normal by means of a 
resistance in the field coil circuit. 

Compound motors are suitable for driving such machines as rock 
crushers which may have to be started up full of rock, because 
they develop the large starting torque with a smaller current than 
the shunt motor, while they drop in speed as the load comes on 
and thereby allow a flywheel connected to the shaft to take the 
peak of the load. 



CHAPTER XVI 
>> LOSSES, EFFICIENCY AND HEATING 

105. Mechanical Losses in Electrical Machinery. — In order to 
keep the armature of an electrical machine rotating, power is 
required to overcome the windage or air friction, the bearing 
friction, and the friction of the brushes on the commutator. This 
power is not available for useful work and is called the mechan- 
ical loss in the machine. 

In a given machine this loss increases with the speed, but at a 
given speed it is practically independent of the load. 

106. Copper Losses. — If R a is the resistance of the armature 
circuit, including the armature winding, the brush contacts, and 
the series field coils then, to force a current T a through this circuit, 
a voltage e a = I a R a is required. This armature circuit drop at 
full-load seldom exceeds 5 per cent, of E t , the terminal voltage. 

The power expended in overcoming this vol- 
tage drop is equal to e a I a = I a 2 R a watts and, 
since this power is not usefully employed, it 
is called the copper loss in the armature circuit. 

If again, R f is the resistance of the shunt 

field coil circuit, Fig. 118, and I f is the shunt 

J -II- • • Fig. 118. 

Current, then the power expended in exciting 

the machine is equal to I/Rf watts where //, which is equal 
to Et/Rf, seldom exceeds 5 per cent, of the current in the ar- 
mature of the machine. 

107. Hysteresis Loss. — Fig. 119 shows an armature which is 
rotating in a two -pole magnetic field. If we consider a small 
block of iron ab then, when it is under the N pole as shown, lines of 
force pass through it from a to b; half a revolution later the same 
piece of iron is under the S pole and the lines of force then pass 
through it from b to a so that the magnetism in the iron is re- 
versed. To continually reverse the molecular magnets of the 
iron in the armature an amount of power is required which is called 
the hysteresis loss in the machine, see page 36. 

The hysteresis loss increases with the number of reversals per 

95 




96 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xvi 



second, that is with the speed, it also increases with the flux 
density. 

108. Eddy Current Loss. — If the armature A, Fig. 119, were 
made of a solid block of iron then, as it rotated, e.m.fs. would be 
induced in the surface layers of the iron and eddy currents would 
flow through the solid mass, see page 55. The power required 

to maintain these currents is called the 
eddy current loss in the armature and 
is kept below 3 per cent, of the arma- 
ture output by lamination of the core, 
see page 58. 

Since the e.m.fs. generated in the 
eddy current circuits depend on the 
rate of cutting lines of force, the eddy 
current loss will increase with the 
speed and with the flux density. 
The total loss in a direct-current machine 




Fig. 119. — Reversal of 
flux in the armature core as 
the armature rotates. 



109. Stray Loss 

consists of 



Stray loss 
Copper loss 



Mechanical losses 
Iron losses 



Windage 

Bearing friction 
[Brush friction 
(Hysteresis loss 
[Eddy current loss 

Ia'Ra 2 + If'Rf 



The term stray loss is used in practice to include the mechanical 
and the iron losses. These losses do not vary with the load so 
that they have practically the same value at no-load as at full- 
load. The stray loss can readily be measured at no-load by 
running the machine idle as a motor at normal speed and normal 
voltage. The motor armature input EJ a , Fig. 120, is then equal 
to the mechanical losses, the iron losses and the small no-load 
armature circuit copper loss, which latter loss may be neglected 
as may be seen from the following problem : 

If a 50-kw., 110-volt, shunt generator requires an armature current 
of 30 amp. when run as a motor at no-load and normal speed and 
voltage, find the stray loss, the resistance of the armature circuit being 0.008 
ohms. 
The armature input at no-load = 110 X 30 = 3300 watts 

= stray loss + I a 2 R a 

= stray loss + (30 2 X 0.008 = 7.2) 
from which the stray loss = 3300 — 7 = 3293 watts. 



Art. 110] LOSSES, EFFICIENCY AND HEATING 97 

In the case of large machines, the stray loss is generally de- 
termined by driving the machine at normal voltage and speed 
by means of a small shunt motor the losses of which are known. 
The machines are connected up as shown in Fig. 121, the generator 
is then excited to give normal voltage E and the input to the 
small motor armature is determined from readings of the voltage 
E m and the current I a . Under these conditions the input to 
the generator must be equal to the windage, friction and iron 
losses of the machine and this input is also equal to E m I a 
minus the motor losses, which latter losses are known. 




Fig. 120. —Machine Fig. 121. — Machine driven by a small motor the 
runs idle as a motor. efficiency of which is known. 

Measurement of the stray loss in a direct- current machine. 

110. The efficiency of a machine = output/input and may be 
calculated from test data as in the following example. 

Draw the efficiency curve for a direct- current flat-compounded generator 
rated at 1000 kw., 600 volts, given the following data 
stray loss = 30 kw. 

R a = the resistance of the armature winding, brush contacts and 

series field coils = 0.006 ohms 
R/ = the resistance of the field coil circuit = 20 ohms 

Affin i ^ i a + 1000 X 1000 ,___ 

At full- load, the load current = w^r = 1666 amp. 

of which /;, the shunt field current = -^r = 30 amp. 

and I a , the armature current = 1696 amp. 

then the stray loss = 30 kw. 

I f 2 R } = 30 2 X 20 = 18 kw. 
I a 2 R a = 1696 2 X 0.006 = 17.2 kw. 
total loss = 65.2 kw. 
generator output = 1000 kw. 
generator input = 1065.2 kw. 
full-load efficiency = 94 per cent. 

The mechanical losses, the iron losses and the shunt field copper loss are all 
independent of the load on the machine and are often classed together as the 

7 



98 



PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xvi 



constant loss, the variable loss being I a 2 R a the loss in the armature circuit, 

so that at half -load, the load current = = 833 amp. 

600 

of which the shunt field current = 30 amp. 
and the armature current = 863 amp. 

then the stray loss = 30 kw. 

Ij 2 R f = 18 kw. 

IJR a = 863 2 X 0.006 = 4.5 kw. 
total loss =52.5 kw. 

generator output = 500 kw. 
generator input = 552.5 kw. 
half load efficiency = 90.5 per cent. 

Other values are worked out in a similar way and the results plotted as in 
Fig. 122. 



LUU 




















60 












40 
20 

































250 



500 750 1000 

Kilowatt Output 



1250 



Fig. 122. — Efficiency curve of a 1000 kw., 600 volt direct-current generator. 

Approximate values for the full-load efficiency of standard 
generators and motors are: 

Kilowatts Full-load efficiency 

1 

5 

25 

100 

500 

1000 

The efficiency of a motor may be determined by loading the 
motor with a prony brake and measuring the total electrical input 
and the corresponding mechanical output. Such a test however 
is rarely carried out except in the case of small motors, the effi- 



80 


per 


cent. 


83 


tt 




88 


u 




91 


CI 




94 


a 




95 


(C 





Art. Ill] 



LOSSES, EFFICIENCY AND HEATING 



99 



ciency of large machines is more readily determined from meas- 
urements of the losses and is worked up as in the last problem. 

111. Heating of Electrical Machinery. — The losses in an 
electrical machine are transformed into heat which causes the 
temperature of the machine to rise above that of the surrounding 
air. The temperature becomes stationary when the rate at which 
heat is generated is equal to the rate at which it is dissipated. 

The rate at which heat is dissipated depends on the difference 
between the temperature of the machine and that of the sur- 
rounding air. During the brief interval after starting under load 
this temperature difference is small, very little heat is dissipated 
and the temperature rises rapidly as shown in Fig. 123. As the 
temperature increases, more of the heat is dissipated and the 
temperature rises more slowly as from b to c. If the load is now 
taken off the machine, the temperature will drop rapidly at first 



50 
°. 40 

to 

f 30 

S 20 

p. 

§ ID 














c 


1 


















^ 


s» 






a> 












V 




















V 


<v 




H 1U 


\a 





















20 .40 60 80 100 120 140 160 180 200 

Minutes 

Fig. 123. — Heating curves of electrical machines. 

and then more slowly as shown in Fig. 123, the temperature drop 
being more rapid when the machine is rotating than when 
stationary because of the better ventilation and the better 
convection of heat . 

112. Permissible Temperature Rise. — Insulating materials lose 
their mechanical and dielectric strengths at high temperatures, 
for example, cotton becomes brittle at temperatures greater than 
85° C. and begins to char at slightly higher temperatures, so 
that-, when cotton is used to insulate machines, the permissible 
rise of temperature is 50° C. above an air temperature of 35° C. 

Methods of insulating have been devised whereby cotton, paper 
and other materials that become brittle and char are not used, 
fireproof materials such as enamel, asbestos and mica being used 
entirely, with such insulation higher temperatures are permissible. 



CHAPTER XVII 
MOTOR APPLICATIONS 

113. Limits of Output. — If the load on a motor is increased, the 
armature current and the armature copper loss both increase and 
the temperature of the machine rises. The maximum load that 
can be put on a motor is that with which the temperature of the 
machine reaches its safe maximum value; a greater load raises 
the temperature to such a value that the insulation of the 
machine is permanently injured. 

The output of a motor is often limited by commutation. When 
interpoles are not supplied, the brushes are shifted from the 
neutral so that commutation takes place in a reversing or com- 
mutating field. Now the effect of armature reaction is to weaken 
this reversing field, see page 84, so that as the armature current 
increases, the reversing field becomes weaker and the motor 
finally begins to spark at the brushes, after which the load can be 
increased no further without injury to the commutator. 

If the commutation limit of output is reached before the 
heating limit, then interpoles may be supplied to improve commu- 
tation, see page 65, and the motor output may be increased until 
the temperature limit is reached. 

114. Open, Semi-enclosed and Totally Enclosed Motors. — 
The cooling of a motor depends largely on the circulation of air 
through the core and windings, so that the frame should be as 
open as possible. 

If chips and flying particles are liable to get into the windings, 
the openings in the frame should be covered with perforated sheet 
metal; the motor is then said to be semi-enclosed. This screen 
throttles the air supply on which the cooling of the machine 
largely depends so that, in order to keep down the temperature 
rise, the output of a motor has to be lower when semi-enclosed 
than when of the open type. 

When a motor has to be totally enclosed, as for open-air service, 
the output of the machine has to be considerably reduced so as to 
keep the temperature down to a safe value thus: 

100 



Art. 115] MOTOR APPLICATIONS 101 

A 10-h.p., 220-volt, 600-r.p.m. motor with 40° C. rise on full-load 
as an open machine can be used to deliver 9 h.p. when semi- 
enclosed and about 6 h.p. when totally enclosed at the same 
voltage and speed and with the same rise in temperature. 

115. Intermittent Ratings. — It was pointed out on page 99 that 
it takes a considerable time for a motor to attain its final tempera- 
ture so that, if a motor has to be operated intermittently for short 
periods, its output may be considerably increased. 

With suitable windings a particular motor frame was given the 
following ratings 

10 h.p., 220 volts, 600 r.p.m. continuous duty 
17 h.p., 220 volts, 600 r.p.m. for 1 hour 
22 h.p.. 220 volts, 600 r.p.m. for 1/2 hour 

the temperature rise at the end of the specified time being the 
same in each case. 

116. Effect of Speed on the Cost of a Motor. — For a given horse-, 
power output, a high speed motor is always cheaper than a slow 
speed motor thus: 

A 10-h.p., 220-volt, 1200-r.p.m. shunt motor weighs 750 lb. and 

costs $150. 
A 10-h.p., 220-volt, 600-r.p.m. shunt motor weighs 1250 lb. and 

costs $250. 
A 10-h.p., 220-volt, 300-r.p.m. shunt motor weighs 1800 lb. and 

costs $350. 
The reason for this is as follows : 

If a given motor frame is supplied with two armatures, one of 
which A has half as many conductors as B but the conductors 
have twice the cross section and can therefore carry twice the 
currrent, then, when run at the same voltage, armature A with half 
the conductors must run at twice the speed of B to give the same 
back e.m.f., but since armature A can carry twice the current 
of B it can therefore deliver twice the output. Thus the armature 
of a 10 h.p., 600 r.p.m. motor could be rewound to deliver 
20 h.p. at 1200 r.p.m., 15 h.p. at 900 r.p.m. or 5 h.p. at 300 
r.p.m. and these armatures would all have approximately the 
same weight and cost. 

117. Choice of Type of Motor. — The characteristic curves of a 
shunt, a series, and a compound motor are shown in Fig. 124, the 
motors having the same torque and speed at full-load. 

The shunt motor takes a current which is proportional to the 



102 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xvii 

torque required, and operates at practically constant speed at all 
loads. It must be noted however that the speed of such a motor 
increases slowly as the field coils heat up because their resistance 
increases and causes the excitation to decrease. If the motor 
has been started cold, the speed may increase 10 per cent, in 3 
hours due to this cause. 

The series motor is the best for heavy starting duty because, for 
any torque greater than the full-load value, it takes a smaller 
current from the line than either the shunt or the compound 
machine. The speed of the series motor decreases rapidly with 
increase of load and becomes dangerously high at light 
loads. For this latter reason, a series motor should always be 
geared or direct connected to the load. 

The compound motor is a compromise between the shunt and 




Armature Current Armature Current 

Fig. 124. — Characteristic curves of direct current motors. 



the series motor. It is better than the shunt motor for. heavy 
starting duty but not so good as the series motor. The speed 
drops somewhat with the load but the motor runs at a safe maxi- 
mum speed even at no-load. 

The service for which each type of motor is suited can best be 
illustrated by a discussion of a few typical motor applications. 

118. A line shaft should run at practically constant speed at 
all loads and so is driven by a shunt motor. The starting torque 
required will seldom exceed 1.5 timea full-load torque and this 
can be obtained with 1.5 times full-load current in the armature, 
which is a reasonable starting overload. 

119. Wood -working machinery such as planers and circular 
saws run at practically constant speed and so are suitably 



Art. 120] MOTOR APPLICATIONS 103 

driven by shunt motors. In the case of heavy planing mills, the 
starting torque required is sometimes excessive due to the inertia 
of the machine, in which case it may be advisable to use a 
compound motor because it requires a smaller starting current 
for the same torque. 

120. Reciprocating pumps, which have to start up against full 
pressure, require a large starting torque, so that although a shunt 
motor is often used for such service yet a compound motor would 
take less starting current from the line. A series motor would be 
suitable so far as starting torque is concerned, but if the suction 
pipe were to leak so that the load on the motor became light, 
then the motor would run away. 

121. Traction Motors. — For traction service, the torque re- 
quired to start and accelerate a car is much greater than that 
required to keep the car moving, so that a series motor is used 
since it is the best for heavy starting duty, see page 91. The 
subject of traction is discussed more fully in Chapter 40, page 
322. 

122. Crane Motors. — The characteristics which make the series 
motor suitable for traction work also make it suitable for crane 
service. The motor is able to develop a large starting torque 
without taking an excessive current from the line; it also oper- 
ates at a slow speed when the load to be lifted is heavy and runs 
at a high speed when the load is light. 

Both crane and traction motors are geared to the load and 
moreover are always under the control of the operator. 

123. Express Passenger Elevators. — An express elevator has to 
be accelerated rapidly, so that a large starting torque is required. 
After the car has moved through about 20 ft., its velocity has 
reached about 500 ft. per min., and has to be kept constant at this 
value. This result is obtained by the use of a heavily compounded 
motor, by means of which a large starting torque is developed 
without an excessive current being taken from the line; when 
acceleration is complete, the series winding is short circuited, and 
the motor operates thereafter as a constant speed shunt machine. 

A series motor would not be suitable for such service because, 
during the rush hours when the car is heavily loaded the motor 
would slow down, whereas at times of light load the car would run 
at an excessive speed, unless specially controlled. 

124. Shears and Punch Presses. — The load curve of a punch 
press is shown in Fig. 125. In order that the peak load may be 



104 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xvii 



carried by the motor without sparking, a motor of about 15 
h.p. would be required, which is much greater than the 
average load of 6.5 h.p. To take the peak load off the 
motor, a flywheel is generally supplied with the press and, in 
order that the flywheel may be effective, the speed of the motor 
must drop as the load comes on. 

A shunt motor is not suitable for such service as it does not drop 
in speed and so does not cause the flywheel to take the load. 

A series motor cannot be used because it would run away when 
the clutch of the press was released, and would probably cause 
the flywheel to burst. 

The motor generally used on large presses is compound wound, 
which motor drops in speed as the load comes on and thereby 
causes the flywheel to give up energy, while the maximum 
speed at no-load cannot exceed a safe value. 




Fig. 125. — Load curve of a punch press. 



Fig. 126. 



A drooping speed characteristic may be obtained from a shunt 
motor by connecting a resistance permanently in series with the 
armature as shown in Fig. 126, the resistance having such a value 
that the voltage drop e r at full-load is about 5 per cent, of E. 
When the load on the motor increases, the voltage drop across 
the resistance increases, that applied to the motor terminals 
decreases, and the speed of the motor drops. When the load on 
the motor decreases, the voltage across the motor increases, the 
speed rises, and energy is again stored in the flywheel. The 
resistance in the power mains supplying the motor may often be 
sufficient to produce this effect. The only objection to the 
method is that an amount of power = e r I a watts is lost in the 
control resistance. 



CHAPTER XVIII 

ADJUSTABLE SPEED OPERATION OF DIRECT-CURRENT 

MOTORS 

For the driving of lathes and other such machine tools, ad- 
justable speed motors are largely used, it is therefore necessary to 
discuss the different methods of speed control before the subject 
of machine tool driving can be profitably taken up. 

125. Speed Variation of Shunt Motors by Armature Control. — 
The speed of a motor may be lowered by decreasing the voltage 





Fig. 127. — Resistance inserted in 
the armature circuit causes the 
speed to decrease. 



Fig. 128. — Resistance inserted in 
the field coil circuit causes the 
speed to increase. 



Methods of adjusting the speed of a direct current shunt motor. 

applied to the motor terminals. This may be done by connecting a 
resistance in the armature circuit as shown in Fig. 127. 

The speed is given by the formula r.p.m. = ky^-^ — — 

where I a R a seldom exceeds 5 per cent, of E a , so that, to obtain 
half speed, the applied voltage E a must be reduced to about 50 per 
cent, of normal, the other 50 per cent, of the line voltage being- 
absorbed by the resistance inserted in the circuit; under these 
conditions, the loss in the resistance, which is equal to e r I a . is also 
equal to the armature input E a I a , and the efficiency of the system 
is less than 50 per cent. The actual efficiency may be figured out 
as in the following problem. 

105 



106 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xviii 

A 10-h.p., 110-volt, 900-r.p.m. shunt motor has an efficiency of 88 per cent., 
an armature resistance of 0.08 ohms and a shunt field current of 2 amp. 
If the speed of this motor is reduced to 450 r.p.m. by inserting a resistance in 
the armature circuit, the torque of the load being constant, find the motor 
output, the armature current, the external resistance and the overall 
efficiency. 

At normal load : 

the motor output = 10 h.p. 

the motor input = 10/0.88 = 11.35 h.p. = 8480 watts 

the total current = 8480/110 = 77 amp. 

the shunt current = 2 amp. 

the armature current = 75 amp., see page 82 

+ i, ■+ 10X33,000 KQK „ . 1 , . ,. nn 

the torque = o^~ vqno = radius, see page 90 

the back e.m.f. = E a — I a R a 

= 110 - (75 X 0.08) = 104 volts, see page 82. 

At half speed: 

mu v. torque X 2tt r.p.m . 

1 he horsepower output = »o nnn and, since the torque 

is constant, the output is proportional to the speed and is equal to 5 h.p. 

The torque = k<f>I a , and, since the torque is constant and so also is the 
excitation, therefore I a , the armature current, is the same as at full speed and 
is equal to 75 amp. 

The back e.m.f. Eb is generated in the armature due to the cutting of lines 
of force and is equal to a const. X<f>X r.p.m., see page 82, and since the flux 
is constant, therefore Eb is proportional to the speed and is equal to 0.5 X 104 
= 52 volts. 

The voltage applied to the motor = Eb + I a R a 

= 52 + (75 X 0.08) 
= 58 volts. 

The voltage drop across the external resistance = 110 — 58 

= 52 volts 
the current in this resistance = 75 amp. 
the resistance = 52/75 = 0.7 ohms, 
the loss in the resistance = 52 X 75 = 3900 watts, 
the total input = 110 X (75 + 2), see Fig. 127, = 8500 watts, 
the motor output = 5 hp. = 5 X 746/1000 = 3.7 kw. 
the overall efficiency = 3.7/8.5 = 44 per cent. 

Since the armature current and therefore the armature copper 
loss have the same value at half speed as at full speed, the torque 
being constant, the temperature rise will be the greater at the 
slow speed because of the poorer ventilation. 

From the above problem it may be seen that, when the speed of 
a motor is reduced by armature resistance, the output is decreased 
and is directly proportional to the speed while the temperature 



Art. 127] ADJUSTABLE SPEED OPERATION 107 

rise, even with this reduced output, is greater than normal 
because of the poorer ventilation. The overall efficiency also is 
exceedingly low, the per cent, loss in the resistance being ap- 
proximately equal to the per cent, reduction in speed, that is, 
being 50 per cent, of the total input at half speed and 75 per cent, 
of the total input at quarter speed. 

126. Speed Variation of Shunt Motors by Field Control. — 
By inserting a resistance in the field coil circuit of a shunt motor, 
as in Fig. 128, the excitation and therefore the magnetic flux are 
reduced and the motor has to run at a higher speed in order to 
generate the necessary back e.m.f., see page 89. 

When interpoles are not supplied, the brushes are shifted from 
the neutral position so that commutation takes place in a reversing 
field, but if the excitation is decreased, then this reversing field is 
decreased and the commutation is impaired; furthermore, the 
higher the speed, and therefore the more rapidly the current in the 
coils is being commutated, the greater is the voltage of self induc- 
tion opposing the change of current and the greater the tendency 
for sparking to take place at the brushes, see page 63. The range 
of speed variation is therefore limited by commutation and an 
increase in speed of about 70 per cent, is about all that can 
generally be obtained by field weakening from a standard motor; 
the flux is then reduced to 1/1.7 = 60 per cent, of its normal value. 
When a greater speed range is required, it will generally be neces- 
sary to use an interpole motor. 

When the speed is controlled by a field rheostat the efficiency 
is not impaired because, as shown in Fig. 128, the control rheostat 
carries only the small shunt current and not the large armature 
current. 

127. Speed Regulation of an Adjustable Speed Shunt Motor. — 
When the speed of a motor varies considerably with change of load 
the speed regulation is said to be poor. When the speed is prac- 
tically constant at all loads the speed regulation is said to be good. 

Suppose that the speed of a shunt motor has been adjusted by 
means of a resistance in the armature circuit, as shown in 
Fig. 127, so as to give a definite speed at a definite load, then, when 
the load is increased, the armature current I a and the voltage e r 
will increase and therefore the voltage E a will decrease and the 
speed of the motor will drop; the speed regulation is therefore poor 
when armature resistance control is used. 

If, for example, this method of control is used when a forging 




108 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xviii 

such as that in Fig. 129 is being turned in a lathe, then the 
motor will slow down when the cut is deep and will speed up when 
the cut is light, so that the speed will be very irregular. 

When the speed of a shunt motor is adjusted by means of a 
resistance in the field coil circuit, as in Fig. 128, the speed 
regulation is good. The speed is given by the formula r.p.m. = 

( F —I J? ) 

kx — - — a a so that since E a is constant, as also is the flux <j> 
<p 

once the field circuit resistance is adjusted, therefore the drop in 
speed between no-load and full-load will seldom exceed 5 per cent., 

since I a R a at full-load seldom exceeds 
5 per cent, of E a . 

128. Electric Drive for Lathes and 
Boring Mills. — Such machines require 
constant horse-power at all speeds of 
the machine spindle so long as the 
amount of metal removed per minute 
remains unchanged. The motor must 
therefore be large enough to develop the necessary horsepower 
at the lowest operating speed without excessive heating. 

The maximum speed of a given motor is limited by centrifugal 
force or by the speed of the gear, while the minimum speed may 
be as low as desired, but when constant horsepower is required at 
all speeds the cost of the motor increases as the minimum speed 
is decreased as may be seen from the following table: 

A. A 10-h.p., 220-volt, 1200-r.p.m. shunt motor weighs 750 lb. 
and costs $150. 

B. A 10-h.p., 220-volt, 600/1200-r.p.m. shunt motor weighs 1250 
lb. and costs $250. 

C. A 10-h.p., 220-volt, 300/1200-r.p.m. shunt motor weighs 1800 
lb. and costs $390. 

the latter motor is necessarily an interpole machine because of the 
large speed variation required, see page 107, and costs about 10 
percent, more than a constant-speed 300-r.p.m. motor of the same 
output. 

The cheapest drive from the point of view of motor cost is that 
obtained by the use of a constant-speed motor such as A in the 
above table, with change gears or coned pulleys to give the 
necessary speed range. If some of the gears are eliminated, and 
a motor such as B in the above table is used, which has a speed 
range of 2 to 1, then the cost of the motor is increased 66 per cent., 



Art. 129] 



ADJUSTABLE SPEED OPERATION 



109 



while for a speed range of 4 to 1 by motor control the cost of the 
motor is 2.6 times greater than if a constant speed motor with 
change gears had been used. 

The speed range of the motor may be obtained by armature 
control, by field control, or by a combination of the two. Arma- 
ture control is used as little as possible because of the low overall 
efficiency of the method and also because of the poor speed 
regulation obtained, see pages 105 and 107. 

129. Multiple Voltage Systems. — When a large number of 
adjustable speed motors are in operation in a machine shop, the 
three wire system shown diagrammatically in Fig. 130 may be 
used with advantage. Two generators in the power house are 
connected in series and three leads a, b and c are taken to each 




QJ 



£j y<y 



A B C 

Fig. 130. — Multiple voltage system. 



adjustable speed motor, the voltage between a and c being 220 
and between a and b and also between b and c being 110 volts. 
To obtain the lowest speed from a motor operating on this 
system, the field coils are connected across the 220-volt mains so 
as to give the maximum flux while the armature is connected 
across either of the 110-volt circuits as shown at A, Fig. 130. 
The speed may then be gradually increased by inserting resistance 
R in the field coil circuit as shown at B. When the speed has 
been doubled in this way, the armature is then connected across 
the 220-volt mains and all the resistance R is cut out of the field 
coil circuit, and the speed may again be gradually increased by 
once more reducing the field excitation by means of the resistance 
R as shown at C. By this means a total speed range of 4 to 1 may 
be obtained without the magnetic flux being reduced at any time 



110 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xviii 

to less than half its normal value, and the efficiency is high over 
the whole range of speed because no resistance is inserted in the 
armature circuit at any time. 

Another multiple voltage system is shown diagrammatically 
in Fig. 131 and is in use in a considerable number of machine 
shops. The voltages available in this case are 90, 160 and 250 
so that a total speed range of 4 to 1 may readily be obtained from 
a standard motor since the flux is never reduced below about 60 
per cent, of its normal value, see page 107. The two-voltage 
system is to be preferred however since 110 and 220 are standard 
voltages and still more so because the two voltages may readily 
be obtained from a single machine called a three-wire generator, 
which is much cheaper than two single 
voltage machines each of half the output. 
This three-wire generator is described on 
page 317. 

130. Ward Leonard System. — One 
method of obtaining a wide speed range, 
without the use of armature resistance, 
would be to use a separate generator with 
each adjustable speed motor and to vary 
the excitation of the generator so as to 
vary the voltage applied to the motor ter- 
minals. Such a system, shown diagrammatically in Fig. 132 is 
called the Ward Leonard System after its inventor. The outfit 
consists of a high-speed motor generator set supplied with each 




T 



90 



,250 



160 



Fig. 131.— Multiple volt- 
age system. 




Motor Generator 

Set 



Adjustable Speed 
Motor 



Fig. 132. — Ward Leonard svstem. 



adjustable speed motor, the set consisting of a high-speed gener- 
ator G direct connected to a motor M. 

To obtain slow speeds, the field excitation of G is reduced so 
as to reduce the voltage E g which is applied to the motor Mi. 
As the field excitation of G is increased, the voltage E g increases 



Art. 131] AD J U ST ABLE SPEED OPERA TION 111 

and with it the speed of the motor M\. The motor Mi can 
readily be reversed by reversing the excitation of G, so that the 
whole control is handled through a small field circuit rheostat 
and the efficiency is comparatively high. 

The Ward Leonard system has been used for printing press 
work to obtain the very low speeds required when the paper is 
being fed into the machine, it has. also been used to secure the 
delicate speed adjustment necessary for operating gun turrets 
in battleships. Because of the ease with which the motor M i 
can be reversed, this system has been used recently for the drive of 
large reversing planers, so as to eliminate the crossed belts. 

131. Drive for Ventilating Fans. — For ventilating purposes it 
is necessary to control the volume of air delivered, to suit the 
requirements. This is done either by reducing the speed of the 
fan or by throttling the orifice. 

When a fan is operated with a fixed orifice, the volume of air 
delivered is proportional to the speed, the pressure is proportional 
to the square of the speed, and the power required is proportional 
to the cube of the speed approximately. 

When the volume of air is reduced by throttling the discharge, 
then the power taken is directly proportional to the volume 
delivered, the speed of the fan being constant. 

For adjustable speed operation the shunt motor is used and, in 
the case of a fan drive, the speed reduction is obtained by the 
armature resistance method of control because of its simplicity; 
the total loss in the controlling resistance being small because 
of the large reduction in the load and therefore in the armature 
current as the speed drops. This may be seen from the following 
example, which is worked out by the same method as that on page 
106: 

The following data is taken from page 106: 

The output is 10 h.p., 110 volts, 900 r.p.m. and the motor is shunt wound, 

the armature resistance = 0.08 ohms 

the exciting current = 2 amp. 

the full-load armature current = 75 amp. 

the back e.m.f. at full-load and full speed = 104 volts. 

If the motor is driving a fan and the speed is reduced to 450 r.p.m. by 
inserting a resistance in the armature circuit, find the motor output, the 
armature current, the external resistance and the overall efficiency. 

The horsepower output is proportional to the cube of the speed approxi- 
mately and is therefore equal to -~ z = 1.25 horsepower. 



112 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xviii 

r™ , horsepower output X 33,000 . . 

The torque = ~ and is therefore equal to full- 

^ 27rr.p.m. M 

, i L 1.25 h.p. 900r.p.m. n ■„ x . . _. _ , 

load torque X 10 , ^4^n = times full-load torque. 

The torque = k<f>I a and, since the flux <f> is constant, therefore the current is 
proportional to the torque, so that the armature current is equal to 0.25 (full- 
load current) or 18.8 amp. 
The back e.m.f. at half speed = 104/2 = 52 volts, see page 106. 

The voltage applied to the motor = Eb + I a R a 

= 52 + (18.8 X 0.08) 
= 53.5 volts 

The voltage drop across the externa] resistance = 110 — 53.5 

= 56.5 volts. 

the current in this resistance = 18.8 amp. 

the resistance = 56.5/18.8 = 3 ohms 

the loss in the resistance = 56.5 X 18.8 = 1060 watts 

the total input = 110 X (18.8 + 2) = 2290 watts 

the motor output = 1.25 h.p. = 1.25 X 746/1000 = 0.93 kw. 

the overall efficiencj' = 0.93/2.29 = 41 per cent. 

so that, although the overall efficiency is still low, the total loss in the 

control resistance is comparatively small. 

132. Armature Resistance for Speed Reduction. — From the 
two problems on pages 106 and 111 it may be seen that the 
resistance required to reduce the speed to 50 per cent, of normal 
may be 0.7 ohms or 3 ohms, the corresponding losses in these 
resistances may be 3.9 kw. or 1.06 kw., and the currents 75 amp. 
or 18.8 amp., depending entirely on the kind of load. 

If a rheostat built originally for constant torque duty is used 
with the same motor for fan operation, it will not have sufficient 
resistance to reduce the speed to the desired value; if a fan duty 
rheostat is used for constant torque service it will have to carry 
a larger current than it was designed for and will burn out. 
It is therefore very essential when specifying armature rheostats 
to specify the type of service for which it will be used. 

133. Motors for Small Desk Fans. — These are usually series 
motors and are connected directly to the line without a starting 
resistance. When the current is switched on, the motor is at 
standstill and its back e.m.f. is zero, but the growth of the cur- 
rent in the machine is opposed by the self induction of the field 
and armature windings which are connected in series, while the 
inertia of the armature and fan are so small that the machine is 
up to speed before the current has time to reach a dangerous 
value. The load on a fan increases as the motor speeds up so 
that the motor cannot run away. 



Art. 134] 



ADJUSTABLE SPEED OPERATION 



113 



134. Printing presses must be run very slowly while being 
made ready, that is, while the web is being threaded, after which 
they must be smoothly accelerated to the desired running 
speed. 

Large presses are equipped with two shunt motors, a main driv- 
ing motor which is direct connected to the driving shaft, and an 
auxiliary starting motor which is connected to the driving shaft 
through a reduction gear and an automatic clutch. 

While the press is being made ready, the auxiliary motor alone 
is connected to the power mains and drives the press at about 
10 per cent, of normal speed. When the press is ready, the main 
motor is connected to the power circuit and is gradually accele- 
rated, and, when the speed of the press has increased sightly above 
the make ready speed, the au- 
tomatic clutch is released due 
to centrifugal force and the 
small motor is thereby dis- 
connected mechanically from 
the press; it may then be dis- 
connected from the power 
mains. 

For small presses, the aux- 
iliary starting motor is dis- 
pensed with, the make ready 
speed being obtained by in- 
serting a resistance in the ar- 
mature circuit. One objec- 
tion to armature control is that the speed regulation is poor, 
see page 107, and, when sufficient resistance is inserted to ob- 
tain 10 per cent, of normal speed, the regulation is very poor 
and the speed of the press is irregular. To obtain low speeds 
which are not irregular, the connection shown in Fig. 133 is used. 
If the current 7 2 is large compared with I a , then a considerable 
change in the value of I a will have comparatively little effect on 
the total current Ii so that the voltage e r will not be greatly 
affected, the voltage E a will therefore remain approximately con- 
stant and the speed regulation will be fairly good. The method 
is not economical since the current 1 2 does no useful work but the 
larger the value of I 2 relative to I a the better is the speed regula- 
tion. This method of control is used only where slow speeds are 
required for short intervals. 




Fig. 



133. — Connection for slow speed 
operation of shunt motors. 



CHAPTER XIX 



HAND-OPERATED FACE PLATE STARTERS AND 

TROLLERS 



CON- 



If a switch is opened in a circuit carrying current, an arc will 
be formed and, unless proper precautions are taken, the switch 
contacts will be burned. 

135. Knife switches such as that shown in Fig. 143 are 
seldom used to open a circuit through which current is flowing; 
they are used to isolate a circuit after the current has been reduced 
to zero. 

The quick-break switch shown in Fig. 134 has a main contact 



T~ T 




Fig. 134. — Quick break type of knife switch. 

blade A and an auxiliary contact blade B held together by 
the spring C. When this switch is opened, the blade B is re- 
tained by friction until A has been withdrawn, the spriug C 
then pulls out the blade B so quickly that no appreciable arc is 
formed. This quick -break principle in different forms is largely 
used when circuits carrying current have to be opened. 

136. Auxiliary Carbon Contacts. — In the switch shown in Fig. 
135, the main contact blocks a and b are bridged by an arch c of 
leaf copper, and an auxiliary carbon contact d is in parallel with 
the contact a. When this switch is opened, the contact a is the 
first to be broken, but the circuit is not interrupted since current 
can still pass through the contact d. This latter contact is broken 

114 



Art. 137] HAND-OPERATED FACE PLATE STARTERS 



115 



when the switch opens further, and an arc is formed which burns 
the carbon tips. Since these tips volatilize without melting, 
they remain in fairly good shape and, when badly burned, can 
readily be replaced; the carbon contacts have the additional 




Fig. 135.— Circuit breaker with auxiliary carbon contacts. 

advantage that by their means a comparatively high resistance 
is inserted in the circuit and the current is reduced before the 
circuit is broken. 

137. Blow-out Coils. — If the conductor ab, Fig. 136, is carrying 




Fig. 136. — Principle of the blow-out coil. 

current and is in the magnetic field NS, it is acted on by a force 
which, according to the left-hand rule, page 7, tends to move it 
upward. If ab is an arc formed between two contacts x and y 
as they are separated, this arc will be forced upward and will 



116 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xix 

lengthen and break. The coil A which produces the magnetic 
field is called the magnetic blow-out coil. 

The application of this principle to a contactor switch is shown 
in Fig. 137. The blow-out coil A produces the magnetic field 
NS, so that if an arc passes between the switch contacts it will be 
forced upward and broken on the contact tips. The polarity of 
the magnetic field must be such that the arc is blown away from 
the switch contacts and not into them. 




A Complete switch 
Fig. 137. 



B Dismantled switch 
-Contactor switch. 



138. Horn Gaps.— The intense heat of an arc causes convection 
currents of air to flow upward so that, if the switch jaws are 
shaped as shown in Fig. 138, the arc stream will be blown upward 
and will finally break between the arcing tips c, which tips may be 
removable. 

This effect may be exaggerated by enclosing the contact in an 



Art. 1421 HAND-OPERATED FACE PLATE STARTERS 117 




/ 



V 



arc chute of such shape that the gases ; expanding suddenly, can 
pass out only through the switch contacts. Such arc chutes, as 
for example that in diagram A, Fig. 137, when combined with 
magnetic blow-out coils, are very effective. 

139. Fuses are used to protect electric circuits from overloads. 
A fuse is a piece of metal of such size and composition that it 
will melt and open the circuit when the current flowing becomes 
large enough to endanger the circuit. 

The melting of a fuse is accompanied by 
an arc and by spattering of the fused metal 
so that it is generally advisable to mount 
the fuse in the- center of a fiber tube and 
surround it with a fireproof powder to 
quench the arc, terminals being supplied as 
at a, Fig. 143, so that the fuse may readily 
be removed and replaced. A 10 amp. fuse 
is expected to carry 12.5 amp. continuously 
and 20 amp. for a period not greater than 
2 minutes. 

140. Circuit Breakers. — Circuits which 
are subject to frequent overloads are gen- 
erally protected by automatic circuit 
breakers such as that shown in Fig. 135, 

rather than by fuses. The operation of such a circuit breaker has 
been described on page 40. 

141. Motor Starters. — The requirements of a motor starter 
have been discussed on pages 85 and 87. These requirements 
have been met in many different ways but it is impossible in the 
space available to describe more than a few standard types. 

142. The sliding contact type of starter, as used for shunt 
motors, is shown in Fig. 109. As the contact arm A is moved from 
the starting position A\ to the running position Ai the following 
operations are performed: 

1. The field coils are fully excited as soon as contact is 
made with the first segment. 

2. The starting resistance is gradually cut out as the contact 
arm is moved from Aito A2', during this interval of time the motor 
should come up to speed. 

3. The contact arm is held in the running position by the no- 
voltage release magnet M. 

To stop the motor, the main switch is opened. This de- 



Fig. 138.— Remova- 
ble horn tips for con- 
tactor switches. 



118 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xix 

energises the release coil M; the contact arm is then pulled 
back to the starting position by the spring S. 

143. Starting Resistance. — The resistance used with a starter 
must have sufficient current carrying capacity to allow the motor 
with which it is used to start up every 4 min. for an hour with- 
out overheating, the load being such that the motor shall come 
up to speed in less than 15 sec. with a starting current not greater 
than 1.5 times full-load current. 

144. Overload Release. — The current in a shunt motor in- 
creases with the load and may damage the machine if the load 








< 


\ <^ 

\\ M 

i — \ n r 




rtwv-il 






> 










1 O 






1 Ly 





( ( 



Fig. 139. Fig. 140. 

Starting boxes with both a no-voltage and an overload release. 



becomes too large. To protect the motor, an overload release 
should be supplied to cut the machine out of operation as soon 
as the load becomes excessive. This overload release may take 
the form of fuses or of a circuit breaker, placed in the circuit as 
shown in Fig. 143, or it may consist of a special attachment 
on the starter- as shown in Fig. 139. 

In this type of starter the contact arm A performs the functions 
of a starting arm while the arm B, used to close the main circuit 
at C is connected to A by the spring S. To operate this starter, 



Art. 145] HAND-OPERATED FACE PLATE STARTERS 



119 



the contact C is closed by the arm B and is held closed by the 
latch L, the arm A is then moved over the contact buttons so as 
to cut out the armature resistance, until it makes contact with 
the no-voltage release magnet M by which it is held againt the 
tension of the spring S. 

The latch L is released by the plunger p which is lifted when 
the line current passing round the solenoid reaches a prede- 
termined value; the arm B then flies up, opens the main circuit 
at C and at the same time deenergizes the magnet M. Before 
the motor can be started again the circuit must be closed at C; 
the spring S at the same time returns the contact arm A to the 
starting position thereby inserting the starting resistance in the 
armature circuit. 

Another type of overload release is shown in Fig. 140. In 
this type, the motor current passes round the magnet and, 



ASKING CONTACTS j 


-■ 


....«.__- 












MAIN CONTACTS ^J 










S~4|f 














e 


f 


n 




abode 

A Complete starter with resistance B Diagrammatic representation 
Fig. 141. — Multiple switch type of starter. 



when it reaches a predetermined value, the arm p is lifted to 
close the contacts tt, the no-voltage release coil M is thereby 
short circuited, the exciting current no longer passes around it, 
and a spiral spring in the hub brings the contact arm A back 
to the starting position and cuts the motor out of circuit. Such an 
overload release is shown on the starter in Fig. 143 and may be 
adjusted to open the circuit for any current up to 1.5 times full- 
load current. This type of release is cheaper than that in Fig. 139 
but it has the disadvantage that it does not protect the motor 
during the starting period. 

145. Multiple Switch Starters. — The sliding contact type of 
starter is liable to give trouble due to arcing at the contacts if 
used for motors larger than 35 h.p. at 110 volts or 50 h.p. at 



120 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xix 

220 to 500 volts; for such service the multiple switch type 
shown in Fig. 141 is to be preferred. 

With this type of starter, each step of the armature resistance 
is cut out by a separate lever and the levers are so interlocked 
that they cannot be closed except in the proper order, thus switch 
b cannot be closed before a because of the stop s. Starting with 
a, the switches are closed hand over hand, each switch as it is 
closed keeping the one to the left of it in contact by means of 
the stops s; the last switch e is held closed by the latch /. Such 
a starter cannot be left partly closed with part of the resistance in 
circuit because none of the switches can stay closed until the last 
switch e is latched. 

When the circuit is interrupted, the magnet M is deenergized, 
the latch / is released, and the switch e opens; the other switches 
then open one after the other. 

146. Compound Starters. — The speed of a shunt motor may 
be adjusted by means of a rheostat in the field circuit (page 
89). When this rheostat is incorporated in the starter, as 
shown in Fig. 142, the resulting piece of apparatus is called a 
compound starter. 

The field contact arm A and the armature contact arm B are 
mounted on the same hub post; there is a spiral spring in the hub 
of B but none in A. To start the motor the two arms, which are 
interlocked, are moved over together, their motion being opposed 
by the spiral spring, and the armature resistance is gradually cut 
out by the arm B which finally makes contact with the no-voltage 
release magnet M by which it is held. The contact arm A is then 
free to move backward over the field contacts thereby weakening 
the shunt field and increasing the speed of the motor. During 
the operation of starting, the field resistance is short circuited by 
the switch s which is kept closed by a spiral spring in the hub h; 
this short circuit is removed while the starting arm B moves on to 
the last contact c, the switch s being then tripped by the projec- 
tions g. 

When the circuit is interrupted, the magnet M is deenergized, 
and the arm B is pulled back by the spiral spring in the hub 
and carries the field arm with it. With such a starter it is im- 
possible to start up except with full field, it is also impossible to 
leave the starting arm B in any position intermediate between 
the starting and the running positions. 

When a motor has to be operated with a weak magnetic field, 



Art. 147] HAND-OPERATED FACE PLATE STARTERS 121 



for adjustable speed operation, it is generally advisable to con- 
nect the no-voltage release coil M directly across the line so that 
its holding power is not weakened when resistance is put in the 
field coil circuit. 





Fig. 142.— Compound starter. 

147. Speed Regulators. — To obtain speeds lower than normal 
a resistance must be inserted in series with the armature, see page 
89. Any of the starters already described can be used as a 
speed regulator if the resistance is able to carry the current of the 
machine continuously without overheating and if provision is 
made to return the contact arm to the starting position should the 
voltage fail. 

A sliding contact type of speed regulator is shown in Fig. 143. 
Resistance in the armature circuit is used to cut down the 
speed below normal, while speeds higher than normal are ob- 
tained by inserting resistance in the field coil circuit on the steps 
between c and d. 

This regulator operates in exactly the same way as a starter of 
the sliding contact type except that the handle can stay on any 
contact and can still be released from that contact by the no- 



122 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xix 

voltage release. Attached to the starting arm is the circular 
ratchet C which moves with the arm and, engaging in the ratchet 
from below, is a pawl which is caused to press against the ratchet 
by the no-voltage release magnet M and to notch into the ratchet 
when the contact arm is on a contact segment. Should the circuit 
be interrupted, the magnet M is deenergized, the pawl is released, 
and the starting arm is returned to the off position by a spring 
in the hub. 




Fig. 143. — Sliding contact type of speed regulator. 

148. Controllers for Series Motors. — Such motors are largely 
used for crane service and the controller used with them must be 
arranged to reverse the direction of rotation by reversing the 
armature connections and must also give speed regulation by 
means of resistance in the armature circuit. 

A simple type of controller for this purpose is shown in Fig. 144. 
The path of the current through the controller is shown in 
diagram A when the motor is running in one direction and in 
diagram B when the direction is reversed. 

The controller shown in Fig. 144 is supplied with a blow-out 
coil A which produces a magnetic field that passes from the front 
arms B to the back arms C, which arms are of iron. This magnetic 
field passes vertically through the slate front and so is at right 



Art. 148] HAND-OPERATED FACE PLATE STARTERS 



123 



angles to the arc formed when a sliding contact leaves a segment, 
it therefore acts to blow out the arc. 



.;l"^^ 








; : ■ 


l— B 














^*v 






/ \ 




A Hoisting B Lowering 

Fig. 144. — Face plate controller for a small reversing series motor. 

The operation of crane and hoist motors is taken up in 
greater detail in Chapter XL. 



CHAPTER XX 
DRUM TYPE CONTROLLERS 

149. Drum type controllers are particularly suited for adjustable 
speed motors which have to be started and stopped frequently, 
because the various operations are performed readily by the move- 
ment of a single handle and take place in their proper order The 
controller is entirely enclosed and can readily be made weather- 
proof, while contact with the live parts is prevented. 

A simple type of drum controller is shown in Fig. 146. It 
consists of a cast-iron drum cylinder A , insulated from a central 
shaft to which the operating handle B is keyed. To this drum, 




Fig. 145. — Developed diagram of machine Fig. 146. — Machine tool con- 
tool controller. troller. 



the copper contact segments a, 6, c, d and e are attached ; these 
are in electrical contact with the drum and therefore with one 
another. The drum carries also a brush contact m which slides 
over stationary field resistance contacts that are mounted on 
the slate C;the contact m is not visible, being hidden by the drum. 
The armature resistance is connected to the stationary fingers 
JiQ,h,j and k which are insulated from one another and mounted 
on a wooden base. 

The action of such a controller may readily be understood 
from Fig. 145 which shows the controller drum developed on to 

124 



Art. 150] 



DRUM TYPE CONTROLLERS 



125 



a plane; the vertical dotted lines indicate the successive positions 
of the contact drum with respect to the row of stationary 
fingers. 

In position 1, the fingers/ and g make contact with segments 
a and b of the drum, and the armature current passes through 
the whole armature resistance, while the field coils are fully excited, 
the exciting current passing through the contact m. In position 
4, the armature resistance is all cut out but the field coils are still 
fully excited. In position 5, the brush m makes contact with field 
segment 5 and the resistance r-i is inserted in the field coil circuit. 
With further motion of the drum from position 5 to position 13, 
the resistance in the field coil circuit is gradually increased, the 
magnetic field is weakened, and the speed of the motor is thereby 
increased above normal. 

150. No -voltage and Overload Release. — The controller 
shown in Fig. 146 is not provided with either a no-voltage or an 





Fig. 147. — No-voltage and Fig. 148. — Connections of no-voltage and 
overload release panel. overload release panel. 

overload release. These are sometimes incorporated in the 
controller but are more often supplied separately on a panel such 
as that shown in Fig. 147, the equipment consisting of a single 
pole magnetic switch A and an overload release coil B. The 
connections of this panel are shown diagrammatically in 
Fig. 148. 

The contact b is kept open by means of a spring. When this 
contact is closed, current passes through the control circuit 
abcdefgh, from the positive to the negative side of the line, and 



126 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xx 

excites the electromagnet M, which closes the switch A and 
allows current to pass to the motor. When the switch A closes, 
the contact s also closes and current now passes through the 
circuit ascdefgh and excites the magnet M, so that the contact b 
can be opened. In this latter circuit is a resistance r, which 
reduces the current in the holding coil M after the switch A has 
been closed so that this switch can open the more readily should 
power go off from the line; this magnetic switch A therefore acts 
as a no-voltage release. 

The total current in the machine passes round the overload 
coil B and, when this current reaches a predetermined value, 
the plunger p is raised to strike the lever q and open the control 
circuit at e, thereby deenergizing the magnet M and allowing 
the switch A to open. - 



21 



21 



AAMNV— 



WW— 




A Motors in Parallel B Motors in Series 

Fig. 149. — Series-parallel system of motor control. 

To stop the motor, the contact d is opened; this opens the 
control circuit and allows the main switch A to open. 

The contacts b and d can be embodied in the controller in 
such a way that the contact b and therefore the switch A cannot 
be closed except when the controller handle is in the off position 
and all the armature resistance is inserted in the armature 
circuit. 

151. Street Car Controller for Series Parallel Control.— 
Street cars are equipped with series motors, and the number of 
motors per car is a multiple of two. To start these machines, a 
resistance is placed in series with the armatures as shown in 
diagram A, Fig. 149, and is then gradually cut out as the motors 
come up to speed. 

The torque required to start and accelerate a car is much 
greater than that required to keep the car in motion so that the 
current I in the motor is large at starting and, if the motors are 



Art. 151] 



DRUM TYPE CONTROLLERS 



127 



connected as shown in diagram A, a large current 27 is taken 
from the line. To reduce this current for half of the starting 
period, the series parallel method of control is adopted. 




Fig. 150. — Street railway controller. 

During the first half of the starting period, the motors are con- 
nected in series, as shown in diagram B, so that the total line 
current passes through both machines; when the starting re- 
sistance is all cut out, each motor has half of the normal voltage 




Series connection 



s c 

Complete diagram of connections 



Parallel connec- 
tion 
Fig. 151. — Diagram of connections of a street railway controller. 

applied across its terminals and runs at half speed. To obtain 
higher speeds, the motors are now connected in parallel and the 
resistance is again inserted in the circuit as shown in diagram A; 



128 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xx 



c£ 



U, 



this resistance is gradually cut out as the motors come up to speed 

and, when it is all cut out, each motor is running on normal line 

voltage. 

These operations are performed with a drum controller of the 

type shown in Fig. 150, which controller is shown developed in 

Fig. 151. 

x y The direction of the cur- 

rent when the controller is in 
position 1 is shown in dia- 
gram A. The current passes 
through the whole armature 
resistance R and then through 
the motors in series. The 
armature resistance is gradu- 
' ally cut out as the controller 

Fig. 152. — Diagram of connections of is moved to position .5, when 
reversing drum. , i , 

& the motors are in series across 

the line and are running at half speed. 

The direction of the current when the controller is in position 8 

is shown in diagram B. The current passes through the whole 

armature resistance and then divides up and passes through the 



c 



t 



O-j 
UJri 

D- 



f^WV 




Fig. 153. 



-Section through drum type of controller, showing the blow out 
coil. 



two motors in parallel. The resistance is gradually cut out as the 
controller is moved to position 12, the motors are then in parallel 
across the line and are running at full speed. 

The complete controller is shown developed in diagram C. 
The student should draw the drum on tracing paper and move it 
over the stationary contacts and note how the various combina- 
tions are obtained. 



Art. 153] D t RUM TYPE CONTROLLERS 129 

152. Reversing Drum. — To reverse the motors, the armature 
connections are reversed relative to the field connections. This is 
done by means of the auxiliary drum B, Fig. 150, which is separate 
from the main drum but is so interlocked with it that the motors 
cannot be reversed except when the main drum is in the off posi- 
tion. This drum is shown developed in Fig. 152. 

When the reversing drum is in position X, current passes 
through the armature from a to b; when in position Y, the current 
passes from b to a. A double set of such contacts are required 
for a pair of motors. , 

153. Mechanical Features of Drum Controllers. — The con- 
troller frame is of cast iron and has an asbestos lined removable 
cover A, Fig. 150; the back of the frame is pierced with a verti- 
cal row of holes lined with insulating bushings through which 
the necessary leads run. 

The contact cylinder D with the copper contacts is supported 
by and insulated from a central shaft to which the operating 
handle is attached. 

The blow-out coil is shown at C. Hinged to the case of the blow- 
out magnet is a steel plate E extending vertically the entire 
length of the cylinder and constituting such a magnetic circuit 
that a powerful magnetic field is maintained across the finger 
contacts as shown in Fig. 153. This steel plate is lined with as- 
bestos on its inner side and carries moulded refractory insulating 
arc barriers F which project between the contact rings. 



CHAPTER XXI 



AUTOMATIC STARTERS AND CONTROLLERS 

The tendency in the operation of electrical machinery is to 
make the starters and controllers self governing so that the 
machinery cannot be injured by careless or unskilled operators. 

154. Automatic Solenoid Starter. — Fig. 154 shows a sliding 
contact type of starter in which the contact arm is moved by 
means of a solenoid. When the main switch K is closed, the 

solenoid A is excited and pulls 
the contact arm upward 
thereby cutting the resistance 
R out of the armature circuit; 
the rate at which the contact 
arm moves may be regulated 
by the dash pot D. 

At the end of its travel the 
contact arm presses on the 
stop s and opens the contact 
c and thereby inserts the re- 
sistance r in the solenoid cir- 
cuit so that the current in 
this circuit is reduced and is 
not larger than necessary to 
hold the arm. in the running 
position. When the power is 
off, or when the switch K is 
opened, the solenoid is deen- 

ergized, and the starting arm falls by gravity to the starting 

position. 

155. Float Switch Control. — With such an automatic starter 
it is not necessary to run the power mains to the point from which 
the motor has to be started. The main switch K may be mag- 
netically operated and placed on the same panel as the starter 
as shown in Fig. 155. This switch is operated by a control cir- 
cuit as shown diagrammatically in Fig. 156. 

130 




Fig 



Automatic solenoid starter. 



Art. 156] AUTOMATIC STARTERS AND CONTROLLERS 131 

When the switch s is closed, the magnetic switch K is excited 
and closes the main circuit so that power is available at the motor 




Fig. 155. — Automatic solenoid starter with main magnetic switch. 

terminals. The switch s may readily be opened and closed by 
means of a float, a pressure gauge or some other device, so as to 
maintain the water level or 
the air pressure in a tank 
within prescribed limits. 

When the float F in Fig. 156 
falls below a certain point, the 
projection e raises the lever I 
and W is moved over. When 
W passes the vertical position 
it drops over and the projection 
g snaps the switch s into con- 
tact and closes the control cir- 
cuit. When the water evel 
reaches the upper limit, the 
projection / trips the lever W 
which opens the switch s and 
causes the motor to stop. 

156. Magnetic Switch Controller. 
are of the multiple switch type, 




Fig. 156.— Float switch. 



-Starters for large motors 



see page 119, and, when the 



132 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxi 

motors have to be operated from a distance, the switches are 
magnetically operated and are closed in their proper order by a 
master controller operating on a control circuit. Fig. 157 
shows such a starter used to control a shunt motor i>y means of 
resistance in the armature circuit. 

The switches A, B, C D, E and F are magnetic contactor 
switches which close when the electromagnets a, b, c, d, e and/ 
are excited. These magnets are connected across the mains 
xy in the proper order by means of a small drum controller M 
called a master controller. 

As the drum M is turned, the first contacts to be made are x 
and 1 and the electromagnet / is excited and the main switch 




Jr-^l 



M 

Master 
Controller 



Fig. 157. — Magnetic switch controller. 



F is closed. In the next position of the drum, the contact 2 is 
closed and the magnet a is thereby excited which closes the 
switch A and connects one terminal of the motor to the line. 
On contact 3 being closed, the magnet b is excited which closes 
the switch B and connects the motor armature across the line 
with all the armature resistance R in series. With further motion 
of the drum M, the resistances R 1} i? 2 .and ' R 3 may be cut out 
one after the other. 

The drum M has to handle only the current in the control 
circuits which current is of the order of 1 amp., the drum may 
therefore be small in size. 

With such a system of control, practically any series of opera- 
tions can be performed in their proper order and several motors 
can be controlled from a single master controller. 



Art. 158] AUTOMATIC STARTERS AND CONTROLLERS 133 

157. Multiple Unit Control of Railway Motors. — The pos- 
sibilities of magnetic switch controllers are well illustrated in 
the multiple unit system of car control. An electric train, made 
up of a number of cars each with its own motors and magnetic 
switch controller, can readily be controlled by a single operator 
at the head of the train. 

The control circuit carries only a small current so that the leads 
are light and flexible; these leads are carried the whole length of 
the train. As each contact of the master controller is closed, 
the corresponding electromagnets under each car are excited 
and the switches closed. If for example, in Fig. 158, the contacts 






Mastei 

Controller 






Fig. 158.— Principle of the multiple unit system of control. 



a and b of the control circuit are energized by the master controller 
then the magnetic switches A on each of four separate cars will 
close simultaneously. 

158. Automatic Magnetic Switch Starters. — With certain 
automatic features attached, the magnetic switch starter may be 
so constructed that the operator has only to close the main switch, 
after which the starting resistance is cut out automatically by the 
controller and the motor is brought up to speed without the 
starting current exceeding a predetermined value. 

The operation of such automatic starters depends on the 
current changes in the armature circuit when the motor with 
which the starter is used is brought up to speed. In the hand 
operated starter in Fig. 159, when the switch A is closed, a current 
of about one and a half times full-load current flows through the 
armature and the starting resistance in series, and the motor 
starts up. As it gains in speed, the back e.m.f. increases and the 
current drops and, when this current has reached full-load 
value, the switch B is closed and the same current cycle is 



134 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxi 

again passed through. Fig. 160 shows the current cycle for a 
four switch starter such as that in Fig. 159. 

In the automatic starter shown diagrammatically in Fig. 161, 
when the switch p is closed, the solenoid a is excited and the 
contractor switch A is closed thereby connecting one terminal of 
the motor to the positive side of the line and at the same time 



'd /c /b /a 



Fig. 159.— Multiple 
switch type of starter. 



Fig. 160. — Current cycle in the 
armature circuit during starting. 



closing the field coil circuit and exciting the machine; the field coil 
circuit is not shown in Fig. 161. 

The switch A, when open, supports the relay e and when A 
closes e is dropped and closes the contacts / so that the solenoid b 
is excited and the switch B is closed thereby connecting the other 




Fig. 161. — Automatic starter with shunt magnetic switches and series relays. 

terminal of the motor to the negative side of the line with all the 
starting resistance in series. Current then passes through the 
armature, R s , R%, Ri, m and q. 

When the switch B closes, a current of about one and a half 
times full-load current flows through the armature, and the motor 
starts up. Now the solenoid g is connected across the resistance 



Art. 159] AUTOMATIC STARTERS AND CONTROLLERS 135 

R, so that the voltage across this coil is the drop across the resist- 
ance, and the current which it sends through the coil is large 
enough to hold up the plunger g even although the mechanical 
support was removed when the switch B closed. As the motor 
gains in speed, its back e.m.f. increases and the drop across the 
resistance decreases so that the current in coil g decreases and, 
when this current has reached a predetermined value, the 
plunger of g drops and closes the contacts h, the solenoid c is 
thereby excited and the switch C is closed cutting out R\, the first 
step of the resistance. 

After B closed but before c was excited, the motor current 
passed through the solenoid m and held up this plunger, but after 
the switch C has closed, this current passes through only the lower 
half of coil m. At the instant C closes, the current is large and 
although passing through only half of coil m it still holds the relay 
open. As the motor accelerates however, the current in the 
armature circuit decreases and finally reaches a value with which 
the solenoid m is no longer able to support its plunger, which 
plunger therefore drops and closes the contacts n, the solenoid d 
is thereby excited and the switch D is closed cutting out R2, the 
second step of the resistance. 

The switch E operates in exactly the same way as D and cuts 
out the last step of the starting resistance. Such a relay switch 
as that used in this type of automatic starter is shown in Fig. 137. 

159. Automatic Starter with Series Switches. — The closing 
electromagnets of the starter shown in Fig. 161 are shunt wound 
and the starter is said to be of the shunt switch type with series 
relays. 

Another type of starter is shown in Fig. 163, the switches in this 
case carry the line current and are called series switches. They 
are so constructed that they will not close wnen the current 
flowing in the exciting coil exceeds a predetermined value. Such 
a switch is shown in Fig. 162. The upper end of the iron plunger 
E carries a non-magnetic stem G to which is attached a copper 
plate H which makes contact with the brushes k when the 
plunger is raised. 

When a small current passes in the coil M, the flux in the 
magnetic circuit passes as shown in diagram A and the plunger 
tends to move upward so as to reduce the reluctance of the 
magnetic circuit. 

When a large current passes in the coil, the flux in the magnetic 



136 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxi 

circuit passes as shown in diagram B; the reduced stem F becomes 
highly saturated so that a large part of the total flux passes across 
the gap l 2 without entering the stem and the plunger tends to 
move upward to reduce the gap h and downward to reduce the 
gap l 2 . The larger the current, the larger the value of fa relative 
to fa + fa, and the less the tendency for the plunger to move up. 
With such a solenoid then, when the current exceeds a pre- 
determined value, the downward pull plus the weight of the 
plunger keeps the plunger from being lifted, but as the current is 
decreased, the flux fa and the downward pull both decrease 





Fig. 162. — Series automatic switches. 



rapidly as the stem F ceases to be saturated, until finally the 
upward pull is able to raise the plunger. 

The critical value of current with which the plunger can be 
lifted may be adjusted by raising or lowering the iron plug C 
so as to change the value of fa relative to that of fa + fa. 

A starter made with three such switches is shown in Fig. 163. 
When the switch A is closed, a large current flows through the 
armature, the starting resistance and the coil Ci in series and the 
motor starts up; the switch g x however does not close until the 
motor has gained in speed and the current has dropped to the 
value for which the solenoid C\ was set. 

When d closes, the first step of the starting resistance is cut 
out and the current, which increased considerably when the 
switch closed, now passes through d and C 2 and the two re- 
maining steps of the resistance. This current decreases as the 



Art. 159] AUTOMATIC STARTERS AND CONTROLLERS 137 

motor speeds up and then C 2 closes the contacts g 2 . The 
same current cycle is again passed through after which C 3 closes 

/wwwwwiwwwvvvwww^ — 




Fig. 163. — Automatic starter with series switches. 

and cuts out the last step of the starting resistance. The con- 
tact # 3 is kept closed by means of the small shunt solenoid s. 





5ERSE5 HELD 



Fig. 164. — Automatic starter with three switches and with the starting- 
resistance. 



The contacts g 3 are the only ones that carry current continu- 
ously because, when C 3 closes, current no longer passes through 



138 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxi 

Ci and C 2 because it has an easier path through the circuit abc, 
the plungers of Ci and C 2 therefore drop. 

When the main switch A is opened, the current in the motor 
circuit drops, but the switch C 3 requires only a small current to 
hold up the plunger which therefore does not drop until the 
current has become almost zero, so that blow-out coils are not 
required to protect the contacts g z . A complete starter is 
shown in Fig. 164. 

The no-voltage and overload release attachments are generally 
supplied on a separate panel such as that shown in Fig. 147, 
page 125, the main switch A being then of the magnetic switch 
type is operated from a push-button circuit. 



CHAPTER XXII 
ELECTROLYSIS AND BATTERIES 

160. Electrolysis. — Certain liquids conduct electricity but in 
doing so they undergo decomposition. Such liquids are called 
electrolytes and include bases, acids and salts, in solution or in 
the molten state. The conductors by which the current enters 
or leaves the electrolyte are called electrodes; that connected to 
the positive line terminal is called the anode and is the one at 
which the current enters, the other is called the kathode. The 
name electrolysis is given to the whole process. 

It would seem that the electrolyte, in addition to containing 
complete molecules of the substance in solution, contains also 
molecules which are dissociated into ions (atoms carrying positive 
or negative charges) and that the metal and hydrogen atoms 
carry positive charges while non metals, the hydroxyl group 
(OH) and the acid radicals (SO*, N0 3 , etc.) carry negative 
charges. If then a difference of potential is established between 
the electrodes, the positively charged ions will be attracted to 
the negative electrode and the negatively charged ions to the 
positive electrode, where they give up their charges. When 
this occurs, the particle or group ceases to be an ion and dis- 
plays at once its ordinary chemical properties. 

If a direct current is passed through a solution of hydrochloric acid (HO), 
using platinum electrodes, then the + H ions will be attracted to the nega- 
tive electrode, where they will give up their charges and then appear as hydro- 
gen gas, similarly the — CI will appear at the positive electrode. 

If a solution of sulphuric acid (H2SO4) is used, then the +H will be liber- 
ated at the negative electrode and the — SO 4 at the positive electrode. 
The SO 4, however, acts on the water of the solution to form sulphuric acid 
and oxygen, which latter gas is liberated while the acid goes into the solution. 
If the positive electrode had been of copper, then the SO4 would have acted 
on the copper to form copper sulphate which would have gone into the 
solution. 

161. Voltameter. — If a negative electrode of paltinum and a 
positive electrode of pure silver are used in a solution of silver 
nitrate (Ag NO3), then silver is deposited on the negative plati- 

139 



140 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxii 

num electrode and the — N0 3 acid radical, liberated at the posi- 
tive electrode, acts on the silver to form more silver nitrate, which 
goes into solution and thereby keeps the concentration of the 
electrolyte constant. 

Faraday's experiments showed that the mass of material 
deposited is proportional to the quantity of electricity (current 
X time) so that the apparatus described above, called the silver 
voltameter, can be used as a measure of quantity of electricity. 
This instrument is used as a primary standard. 

162. Electric Battery. — An electric battery is a device for 
transforming chemical energy into electrical energy and consists 
essentially of two dissimilar plates in a solution which acts more 
readily on one plate than on the other. A difference of potential 
is found between two such plates so that if they are joined by a 
wire an electric current will flow in this wire. The magnitude of 
this e.m.f. depends only on the material of the plates and on the 
electrolyte and, for a given pair of plates, is independent of their 
area. 

There are innumerable types of battery, but in nearly every 
case one plate is of zinc and the other of either carbon or copper. 
If a battery is made of copper, zinc and dilute sulphuric acid, it 
will be found that, when current flows in a conducting wire con- 
necting the copper and the zinc plates, the zinc goes into solution 
as ZnS0 4 while hydrogen is given off at the copper plate. The 
zinc and the sulphuric acid are therefore used up and electrical 
energy is obtained at the expense of the chemical energy which 
was contained in these materials. 

When the cell becomes exhausted, the zinc plate and the elec- 
trolyte have to be renewed. When fresh materials are used, the 
battery is called a primary battery; when the materials are re- 
newed by electrolysis in a way that shall be described later, the 
battery is called a secondary or storage battery. 

163. Theory of Battery Operation. — If we consider a battery 
made up of copper, zinc and dilute sulphuric acid, then the essen- 
tial difference between these metals so far as battery operation is 
concerned is that the zinc is the more readily acted on by oxygen 
or has the greater chemical attraction for oxygen so that, while 
both copper and zinc attract the —0 ions in the solution, the 
attraction of the zinc is the greater. As both metals combine 
with the attracted oxygen they become negatively charged and 
soon repel the negative oxygen ions as strongly electrically as 



Art. 166] ELECTROLYSIS AND BATTERIES 141 

they attract them chemically. When equilibrium is established, 
both metals are negatively charged but the negative charge on the 
zinc is the greater and its potential is therefore the lower. If the 
two metals are now joined by a connecting wire outside of the 
solution, electricity flows from the copper to the zinc and the 
plates tend to come to the same potential, so that the state of 
equilibrium is disturbed; the potential of the zinc rises slightly 
above its potential of equilibrium so that it is able to attract 
more negative oxygen, while the potential of the copper falls 
slightly below its potential of equilibrium so that it now attracts 
the positive hydrogen ions, the voltage between the plates is 
therefore maintained and so also is the current in the conducting 
wire. • 

The zinc oxide formed is acted on by the sulphuric acid to form 
zinc sulphate which goes into the solution and is no longer an 
active constituent of the cell. 

164. Polarization. — The action of this battery weakens after a 
few minutes of operation, and this weakening is found to be due 
to a layer of hydrogen bubbles which cling to the copper plate 
after giving up their charge. The cell is then one which has ac- 
tive plates of hydrogen and zinc and gives a much lower e.m.f. 
than one of copper and zinc. Hydrogen has also a high electrical 
resistance so that the current that can be drawn is small. This 
defect of the battery is called polarization, and different methods 
are used to keep the hydrogen bubbles away from the copper 
plate, generally by the use of a depolarizing substance containing 
an excess of oxygen, which substance is placed around the Copper 
plate. 

165. The E.M.F. and Resistance of Cells. — The electromotive 
force depends only on the materials of the cell and is independent 
of their size, shape or arrangement. 

The internal resistance of a cell of given materials is proportional 
to the distance between the plates and inversely proportional to 
their area and, in order that a cell may have a low internal resist- 
ance, the plates should have a large surface and should be close 
together. 

166. The Daniell cell is a commercial type of copper, zinc and 
sulphuric acid battery largely used for telegraph work. The 
depolarizing substance in this cell is copper sulphate (CuS0 4 ). 
In one form of this battery the sulphuric acid with the zinc are 
placed in a porous pot and this in turn is placed in a saturated 



142 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxii 

solution of copper sulphate contained in a glass jar, the copper 
plate is immersed in this latter solution. By means of the porous 
pot the liquids are kept from mixing but the action of the battery 
is not impaired since the ions pass freely through the walls of the 
pot. 

The hydrogen liberated at the copper electrode acts on the 
adjoining copper sulphate to form copper and sulphuric acid; the 
copper is deposited on the electrode so that there is no hydrogen 
layer formed and therefore no polarization of the cell until all 
the copper sulphate in the solution has been exhausted. 

The usual size of Daniell cell has a voltage on open circuit of 
about 1.1 and an internal resistance of about 2 ohms so the largest 
currenUthat can be obtained from such a cell is 0.55 amperes on 
short circuit. 

167. Calculation of the E.M.F. of a Daniell Cell.— Faraday's 
experiments showed that, when electrolysis takes place, the num- 
ber of gm. of substance separated out per coulomb is equal to 

atomic weight . ■ 

, or the coulombs required to produce a number 

96,540 X valency 

of grams equal to the atomic weight is 96,540 X the valency. 

Copper (Cu) and zinc (Zn) have a valency of two in compounds 
such as CuS0 4 and ZnS0 4 , because one atom of Cu or Zn replaces 
two atoms of H from H 2 S0 4 , and, since the atomic weight of Cu 
is 63.75 and that of Zn is 65.37 therefore 96,540 X 2 coulombs 
will separate out 63.75 gm. of Cu from CuS0 4 and 65.37 gm. 
of Zn from ZnS0 4 . 

When 63.75 gm. of Cu are formed into CuS0 4 , 197,500 gm. 
calories of energy are liberated and when 65.37 gm. of Zn are 
formed into ZnS0 4 , the energy liberated is 248,000 gm. calories. 

In a Daniell cell then, when 65 . 37 gm. of Zn have been con- 
sumed, 96,540 X 2 coulombs have passed and 63.75 gm. of Cu 
have been separated out from the CuS0 4 , the energy given up 
by the cell must therefore be 

248,000 - 197,500 = 50,500 gm. calories 

= 50,500 X 4.186 watt sec. see page 15 
= 211,400 watt sec. 
and 96,540 X 2 X voltage of cell = energy obtained 

= 211,400 watt sec. 

211 400 

from which the voltage of the cell = ' = 1.1 volts 

96,540 X 2 

assuming that no energy was lost in the form of heat. 



Art. 170] ELECTROLYSIS AND BATTERIES 143 

168. Local Action. — In a well-designed and constructed cell, 
action takes place only when the cell is delivering energy. Com- 
mercial zinc, however, dissolves in sulphuric acid even when the 
external circuit is not closed because the zinc is impure and local 
action is set up between the impurities and the zinc and a number 
of small internal batteries are formed in which the zinc is con- 
sumed but no voltage is available at the terminals. To prevent 
this action the zinc is amalgamated, that is covered with a layer 
of mercury. To amalgamate zinc, clean it with sandpaper, then 
immerse in dilute sulphuric acid and while still wet apply mercury 
to it with a rag. So far as the action of the battery is concerned 
the mercury is inert. 

169. Leclanche* Cell. — This battery consists of carbon (C) 
and Zinc (Zn) in ammonium chloride (NH 4 C1). The zinc is 
converted into zinc chloride (Zn CI) while ammonia (NH 3 ) and 
hydrogen (H) appear at the carbon plate. To prevent polariza- 
tion, the carbon is surrounded with an oxidizing agent in the form 
of manganese dioxide (Nn0 2 ), the oxygen of which attacks the 
hydrogen to form water. In the usual form the carbon is placed 
in a porous pot and is surrounded with granules of manganese 
dioxide and of carbon. The action of this depolarizer is slow 
so that, when used to supply current for a considerable time, the 
cell becomes polarized and runs down; it will recover, however, 
if left on open circuit. 

The battery gives a voltage of about 1.5 on open circuit and 
has an internal resistance of about 1.5 ohms. It is much used for 
bell ringing and other intermittent work and requires little atten- 
tion beyond the addition of water as the solution evaporates, and 
the renewal of the zinc as it is used up. The zinc generally sup- 
plied is not amalgamated, and non-conducting crystals of ZnCl 
stick to its surface instead of dropping to the bottom; the opera- 
tion is improved if amalgamated zinc is used. 

170. Dry Cells. — These are largely used when the battery is 
subject to motion, as in motor cars and motor boats. There are 
innumerable types but nearly all consist of carbon, zinc and 
sal ammoniac with other ingredients, in fact they are Leclanche 
cells. The form usually taken consists of an outer cell of zinc 
which is one electrode and is lined with blotting paper. The car- 
bon stick is placed in the center and is surrounded with a mixture 
of manganese dioxide and powdered carbon. The blotting paper 
and the powdered mixture are saturated with the electrocute and 



144 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxii 

a layer of sawdust is placed on the top after which the cell is 
sealed with pitch. The outside is then wrapped with paper so 
as to insulate the zinc. The current depends on the size of the 
cell, the usual size, 2.5 in. dia. and 6 in. high, will give about 
15 amperes on short circuit for a few seconds. 

Such cells deteriorate in storage, to prevent which one type has 
a hollow carbon rod with a stopper; it is shipped dry and so is 
chemically inactive until ready for use when it is filled with water 
through the hole in the carbon. 

These cells run down as the active materials are consumed 
but may often be recuperated temporarily by the addition of 
sal ammoniac to the blotting paper through a hole made in the 
pitch, even water will help in an emergency. 

171. Edison Lalande Cell. — The active materials in this cell 
are copper oxide (CuO) and zinc in a solution of caustic potash 
KOH. The oxygen ions combine with the zinc to form zinc 
oxide and this combines with the potash to form a soluble salt. 
The hydrogen reduces the copper oxide to metallic copper and so 
does not polarize the cell. The construction is very rigid. The 
positive plate of CuO and the negative Zn plate are separated 
by porcelain spacers and rigidly fastened to the top of the con- 
taining jar which jar is of enameled steel. A layer of mineral oil 
is placed on the top of the electrolyte to keep out the air and to 
prevent the formation of creeping salts. 

The internal resistance of this cell is low so that large currents 
can be drawn from it; thus a 600 ampere-hour battery has an 
open-circuit voltage of 0.95 and an internal resistance of about 
0.02 ohm; this battery is designed to deliver 7 amp. for 85 hours 
although the battery would give over 40 amp. on short circuit. 

172. Power and Energy of a Battery — The active materials 
in a battery contain a definite quantity of chemical energy 
which may be transformed into electrical energy, so that a bat- 
tery can give a definite number of watt-hours or a definite num- 
ber of ampere-hours at normal voltage. 

This energy may be taken as a small current for a long time 
or a large current for a short time so that while the total energy 
in the battery is fixed by the weight of the active materials the 
power of the battery (volts X amp.) may vary over a wide range. 

173. Battery Connections. — If E is the open-circuit voltage 
of a battery, R b is the internal resistance and R is the resistance 



Art. 173] ELECTROLYSIS AND BATTERIES 

of the external circuit then the current I = 



E 



maximum value on short circuit = 



Rb + R 



145 

and has a 



If n batteries are connected in series, then the current / = 
and an increase in the number of batteries does not 



nR b + R 

produce any considerable increase in the current unless R is large 

compared with R b . 

If n batteries are connected in parallel, then the current I = 

E 
h— ~, — ■ — ^ and an increase in the number of batteries does not 
Rb/n -\- R 

produce any considerable increase in the current unless R is 

small compared with R b . 

The internal resistance of a Daniell cell is 2 ohms and the no-load voltage 
is 1.1 volts. The current when the batteries are connected in series and 
in parallel is given in the following table (a) when the external resistance 
is 10 ohms and is greater than that of the battery; (b) when the external 
resistance is 1 ohm and is less than that of the battery : 



Total 
Number of cells resistance 
of cells 



External 
resistance 



Terminal voltage 



Amperes 



Open circuit 



With 
current 



(a) 


2 ohms 


10 ohms 


0.092 


1.1 


0.92 


10 in series 


20 ohms 


10 ohms 


0.365 


11.0 


3.65 


10 in parallel 


. 2 ohm 


- 10 ohms 


0.108 


1.1 


1.08 


(b) 


2 ohms 


1 ohm 


0.365 


1.1 


0.365 


10 in series 


I 20 ohms 


1 ohm 


0.52 


11.0 


0.52 


10 in parallel 


. 2 ohm 


1 ohm 


0.92 


1.1 


0.92 



10 



CHAPTER XXIII 
STORAGE BATTERIES 

174. Action of the Lead Cell. — If a plate of lead peroxide 
(Pb0 2 ) and one of lead (Pb) are placed in a solution of 
sulphuric acid (H 2 S0 4 ) a battery is formed with the peroxide 
plate at the higher potential. The generally accepted theory of 
operation is as follows : 

The sulphuric acid acts on the lead plate to form lead sulphate, 
which material stays on the plate 

H 2 SOi + Pb = PbS0 4 + H 2 

The hydrogen ions have their charge neutralized at the peroxide 
plate which they reduce to lead monoxide 

H 2 + Pb0 2 = PbO + H 2 

The lead monoxide thus formed is acted on by the acid to form 
lead sulphate, which material stays on the plate 

PbO + H 2 S0 4 = PbS0 4 + H 2 

The final result may therefore be represented by the equation 

Pb + Pb0 2 + 2H 2 S0 4 - 2PbS0 4 + 2H 2 

so that, during discharge, sulphuric acid is taken from the elec- 
trolyte, water is added to it, and the specific gravity of the 
electrolyte is thereby decreased, while both plates are converted 
into lead sulphate. 

If now current from some external source is forced through 
the cell in the opposite direction so as to cause electrolysis, 
the action is completely reversed: 

The positive H ions of the acid are attracted to the negative 
PbS0 4 and reduce it to lead 

H 2 + PbS0 4 = Pb + H 2 S0 4 

The negative S0 4 ions of the acid are attracted to the positive 

146 



Art. 176] STORAGE BATTERIES 147 

PbS04 and, being liberated there, act on the water of the solution 
to form sulphuric acid and oxygen 

2S0 4 + 2H 2 = 2H 2 S0 4 + 2 

This oxygen, with more water from the electrolyte, act on the 
sulphate plate to form peroxide of lead and sulphuric acid 

2 + 2H 2 + 2PbS0 4 = 2Pb0 2 + 2H 2 S0 4 

The final result may therefore be represented by the equation 

2PbS0 4 + 2H 2 = Pb + Pb0 2 + 2H 2 S0 4 

so that during charge the plates are reformed while sulphuric 
acid is added to the electrolyte, water is taken from it, and the 
specific gravity of the electrolyte is thereby increased. 

After the plates have been completely reformed, further charg- 
ing will cause the hydrogen and oxygen to appear as gases which 
bubble up through the electrolyte from the surfaces of the plates, 
the cell is then said to be gassing. 

175. Storage or Secondary Battery. — -An electric battery which 
can be reformed by chemical means is called a storage or sec- 
ondary battery. There is no essential difference between a 
primary and a secondary battery. In the former the active 
materials themselves are renewed when the cell is exhausted, 
whereas the latter is designed to permit of the materials being 
brought back to their original state by electrolysis, that this 
may be possible, no product formed during discharge must be 
lost. 

176. Sulphation. — There would appear to be two forms of lead 
sulphate, an unstable electrolytic form which is readily reduced 
by an electric current, and the lead sulphate formed by chemical 
precipitation which latter is a non-conducting substance not 
decomposed by an electric current. This latter substance must 
not be allowed to accumulate on the plates. 

The electrolytic form changes slowly into the insoluble form 
and for that reason a lead battery must not be left discharged for 
any length of time; insoluble sulphate also tends to form if the 
battery is discharged too far. 

The formation of this insoluble sulphate is called sulphation 
and must be prevented by proper operation of the battery. If 
sulphation has commenced on some of the plates and has not 
gone too far, the plates may be cleared by overcharging the 



148 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxiii 

battery for a long time, the hydrogen and oxygen formed when 
the cell is overcharged tear off the insoluble sulphate. 

177. Construction of the Plates.— When fully charged, the posi- 
tive plates are chocolate in color and the peroxide is hard while 
the negative plates are gray in color and the spongy lead is so soft 
that it can be scraped off with the finger nail. During discharge, 
these plates are converted into lead sulphate, which is bulky, so 
that the plates expand and, unless carefully designed, are liable 
to buckle, especially if the cell is discharged rapidly so that the 




Fig. 165. — Plante plate, showing cross section. 



sulphate is formed rapidly and loosely. The negative plate in 
addition must be so designed that the soft material will not be 
readily washed off. 

There are two ways of forming the active material. By the 
Plante process the material is formed electrochemically out of the 
lead plate itself, the plates being grooved or made in the form of a 
grill so as to have a large exposed surface. The plates shown in 
Fig. 165 are made out of pure rolled lead passed backward and 
forward through grooving rolls which spin the lead into ribs the 
pitch of which is made to suit the service. The peroxide and 
spongy lead, formed electrochemically on the positive and nega- 



Art. 178] 



STORAGE BATTERIES 



149 



tive plates respectively, pack into the grooves and are tightly held 
in the narrow spaces. 

Pasted plates are made by spreading a paste of the active 
material on to a supporting grid of lead hardened with a small 
quantity of antimony, this being the only commercially available 
material which will resist the action of sulphuric acid and will not 
set up local action with either the spongy lead or with the 
peroxide. Most of the processes by which these plates are pre- 
pared are secret, but by such processes plates can be made soft 
and porous or hard and dense according to the service for which 
they are required. 

Plante plates are used largely for stationary batteries; they are 
heavier and more costly than pasted plates but are also more 




Positive group Negative group. 

Fig. 166. — Groups of plates for a lead battery. 



durable and less liable to lose active material by rapid charging 
and discharging. For automobile and motor truck service, 
pasted plates are generally used because they are lighter than 
Plante plates. 

178. Construction of a Lead Battery. — To obtain large capacity 
from a battery, a large surface must be exposed to the electrolyte, 
and, since the size of a single plate is limited, increased capacity 
must be obtained by connecting a number of plates in parallel to 
form a group as shown in Fig. 166, there being one more plate in 
the negative than in the positive group. Two sets of plates are 
then sandwiched together, adjoining plates being separated from 
one another by glass rods in the case of large power-house cells or 



150 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxiii 

by wooden and rubber separators in smaller cells. The wooden 
separators, see Fig. 167, are specially treated and are grooved 
vertically to allow the gases and the electrolyte to circulate freely; 
the flat side is placed against the soft negative plate while between 
the positive plate and the corrugations on the wood a sheet of 
perforated hard rubber is placed, as shown in Fig 167, which helps 
to prevent washing out of the active material. 

The plate groups are placed in an acid-proof tank, generally 
of glass, hard rubber, or of wood lined with lead, and are sup- 



P05ITIVE STRAP/type'i" 
■ ^ Cov 
Cross Bar , 

(PARTOF5TRAP) 

Hold-down 
Plate Lu& — : 

Positive Plate 

hardRubser' 

fAGA»M5TP02S T :ve =LftTE]~— - 

Wood Separator 

(n.AT5:Ci-CA-.;-.'.r-3A-v; 
, v PiA"e G°>'vi:. ; ; D£.i- 

&A-.5T ?j£2; c i-t"j 



NE&ATivE Plate 



NEGAT' 




HaRDHUBBERJAR 



Fig. 167.— Portable type of lead battery. 



ported in various ways as shown in Figs. 168 and 169, ample 
space being left below the plates for the accumulation of sedi- 
ment which must not be allowed to short circuit the plates. 

To minimize leakage of electricity, the tanks are insulated from 
one another. Small cells are generally carried on shallow trays 
filled with sand and supported on glass insulators as shown in 
Fig. 168. Large lead-lined tanks are generally mounted as shown 
in Fig. 169 with a double set of insulators between the tank and 
the ground. 



Art. 179] 



STORAGE BATTERIES 



151 



To prevent loss of electrolyte due to spraying when the 
cells are gassing freely, glass sheets are placed over the tanks. 

For automobile work, hard rubber jars are used. These are 
placed in a wooden box and compound is poured around the 
jars and flooded over the top so as to hold them securely and 
also to seal the battery and thereby prevent loss of electrolyte. 
To provide for the escape of the gases generated during over- 
charge, vents are provided, constructed so as to allow the gases 
to escape but prevent the escape of the electrolyte. 




Fig. 



168. — Small cell in a glass jar Fig. 
supported on sand. 



169. 



-Central station type of 
lead cell. 



179. Voltage of a Lead Battery. — The terminal voltage of a 
battery 

= E b + IR while the battery is charging 
= E b — IR while the battery is discharging 
where E h is the internal generated voltage of the battery 
I is the current in amperes 
R is the effective internal resistance in ohms 
so that the larger the current the greater the difference between 
the charge and discharge voltages. 



152 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxiii 



The value of E b , the generated voltage, depends on the strength 
of the electrolyte and increases slightly as the acid becomes 
stronger, it therefore increases during charge and decreases 
during discharge. 

Tl e internal resistance of the cell depends on the specific 
gravity of the electrolyte and is a minimum when this has a 
value of about 1.22, which is about the normal value used in 
cells. During discharge the acid becomes weak, particularly in 
the pores where the action is taking place, and sulphate is formed 
which is a comparatively poor conductor of electricity, so that, 
towards the end of discharge, the internal resistance of the cell 
increases rapidly. During charge the acid becomes strong, 
particularly in the pores, so that toward the end of charge the 
internal resistance again rises. 



3.0 



2.0 



1.0 



Charged at 32 Amp. 













b 




^.d 




/. 
































^ 


„ 70 
a 

tig 


^r 


c 


42.5 


Amp. 


e 




29 An 


P. 































































.5 

Houi 



Fig. 170. 1 - 



-Charge and discharge curves of the same lead cell at different 
discharge rates. 

The curves in Fig. 170 show how the voltage varies during 

charge and discharge, the charge being continued until the voltage 

and the specific gravity have become constant and the active 

materials therefore completely formed, while the discharge is 

stopped when the terminal voltage has dropped to about 1.8, 

the safe minimum value at the normal discharge rate. If the 

discharge is carried much further than this it becomes difficult 

to clear the plates of sulphate on re-charging. 



The ratio 



average voltage on discharge Eb — IR . 



- = -^ — t-t— is called the 



average voltage on charge Eb + IR 
volt efficiency and is lower the larger the current I, that is, the 
higher the rate of charge and discharge. At the normal rates of 
Secondary cells by Aspinall Parr; Journal of the Inst, of Elect. Eng., 
vol. 36, p. 406; sec. 1905. 



Art. 181.] STORAGE BATTERIES 153 

charge and discharge this efficiency is seldom less than 80 per 
cent. 

180. Capacity of a Cell. — The current that can be drawn from a 
cell depends on the plate surface exposed to the electrolyte, on 
the porosity of the plates, and on the rate of discharge. An 
excessive current is liable to buckle the plates, see page 148, while 
a short circuit will cause such violent action and such a sudden 
evolution of gas in the body of the active material as to cause 
parts of this material to be ejected from the plates. 

The capacity of a battery is given in ampere-hours at a definite 
rate of charge and discharge, generally an 8-hour rate, except 
in the case of automobile batteries which are generally rated at a 
4-hour rate as this more nearly corresponds to the actual service 
conditions. A battery with a rating of 100 amp. -hours will 
deliver 12.5 amp. for 8 hours before the voltage drops below 
1.8. Theoretically this same battery, after being fully charged, 
should give the same total quantity of electricity at all rates of 
discharge, that is, it should give 25 amp. for 4 hours or 50 
amp. for 2 hours, but it is found that the ampere-hour capacity 
decreases as the discharge rate is increased as may be seen from 
the curves in Fig. 170. This particular battery had a capacity of 

29 amp. for 10 hours or 290 amp. -hours at a 10-hour rate, 

42.5 amp. for 6 hours or 255 amp. -hours at a 6-hour rate, 

70 amp. for 3 hours or 210 amp.-hours at a 3-hour rate, 

134 amp. for 1 hour or 134 amp.-hours at a 1-hour rate. 

The cause of this loss of capacity at high rates of discharge 
is that the active acid in the pores becomes used up in forming 
sulphate, so that only water is left in the pores since the acid 
is not replenished fast enough by diffusion from the bulk of acid 
in the tank, the action is therefore limited to the surface layers 
when the rate of discharge is high and does not penetrate into 
the active material. 

181. Ampere-hour Efficiency. — Test data is given in Fig. 170 on 
a particular battery which, after being completely charged, was 
able to deliver 134 amp. for 1 hour before the voltage had 
dropped to 1.7. To recharge this battery required 32 amp. 
for 5 hours or 150 amp.-hours before the voltage and the specific 
gravity had become constant. The reason for this additional 
quantity of electricity is that, toward the end of a charge, hydro- 
gen and oxygen gases are given off and these require a definite 



154 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxiii 



quantity of electricity for their formation, which electricity is 
not available on discharge because the gases have escaped. 

If the charged battery is now discharged at 42.5 amp. the 
voltage does not drop to 1.75 until after 6 hours and the output 
is 42.5 X 6 or 255 amp. -hours. The action being slower has 
gone deeper into the plates and more active material has been 
turned into sulphate, but, just because of this increased depth of 
action, it is not advisable to allow the voltage to drop so far. 
To recharge this battery now requires 32 amp. for 9 hours or 
288 amp. -hours. 

The number of ampere-hours required to charge a battery is 
greater than the number taken out during the previous discharge 
ampere-hours output 



and the ratio 



is called the am- 



ampere-hours input to recharge 
pere-hour efficiency of the battery. 

From the test curves in Fig. 170 the following results are 
determined : 



Discharge 



Charge 



Amp. 



Hours 



Amp.- 
hours 



Amp. 



Hours 



Amp.- 
hours 



Amp.-hour 
efficiency, 
per cent. 




From these figures it may be seen that the higher the rate of 
discharge the lower the ampere-hour efficiency, the charging 
rate being the same in each case. The reason for this is that when 
the discharge rate is high the output is small so that the useful 
input of the next charge is small and the portion used in forming 
gases is proportionately large. 

182. Watt-hour efficiency. — This quantity 

watt-hours output 



watt-hours input to recharge 
_ amp. -hours output X average discharge voltage 

amp. -hours input X average charging voltage 
= amp.-hour efficiency X volt efficiency 

both of which quantities are lower, the higher the rate of charge 
and discharge. 

With floating batteries, which are used to carry peak loads 



Art. 184] 



STORAGE BATTERIES 



155 



of short duration and are not charged and discharged completely, 
there is little or no gassing and the ampere-hour efficiency is 
almost 100 per cent., while the peak voltages at the end of a 
charge and the very low voltages at the end of a discharge are 
avoided and the volt efficiency is high. Thus, while a battery 
in a central station supplying a lighting load of several hours' 
duration will have an average watt-hour efficiency over a period 
of 12 months of 74 per cent., a similar battery in a central station 
supplying a traction load, the load on the battery being inter- 
mittent, will have an average watt-hour efficiency of 84 per cent. 
over a period of 12 months. 

183. Effect of Temperature on the Capacity. — In general, the 
cooler a battery is kept the longer is its life but the lower the 
capacity. Lowering the temperature of the electrolyte increases 
its internal resistance and causes an increased drop for a given 
current. The curves in Fig. 171 show how large is the drop in 
capacity when the temperature is lowered; the temperature was 



2.0 



1.0 



^T^ 20°OT- 40°C. 









































20 



30 40 
Jlinute.s 



Fig. 171. * — Discharge curves of a lead cell with the same current but at differ- 
ent temperatures. 

maintained constant during any one test by means of circulating 

water. 

The salvation of a battery in cold weather lies in the fact that 
it is self warming, the internal resistance and therefore the 
PR loss become greater as the temperature decreases. For 
electric-truck work in cold climates it is advisable to lag the 
batteries, or at least to place them in a wind-proof compartment. 
The temperature on charge should be limited to about 40° C; 
continual operation at higher temperatures tends to reduce 
the life of the cell.. 

184. Limit of Discharge. — As pointed out on page 152, a 
battery should not be allowed to discharge to a voltage below 
about 1.8 because there is then an excess of sulphate formed on 

*Liagre, L'Celairage Electrique, Vol. 29, p. 150; Nov. 2, 1901. 



156 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxiii 



the plates and a tendency for the plates to buckle and to sulphate 
permanently. The amount of charge in a battery is best deter- 
mined by measurements of the specific gravity of the electrolyte 
since this gives a measure of the amount of acid that has gone 
to form sulphate on the plates. The specific gravity may 
readily be measured by a hydrometer of the type shown in 
Fig. 172. 

The voltage of a battery is not an accurate index of its condition 
because the voltage depends largely on the rate 
of discharge. Voltage readings on open circuit 
are of no value because this voltage is almost 
independent of the amount of charge still in the 
battery. 

185. Treatment of Lead Cells. — From a study 
of the action of lead cells, the treatment they 
should receive may be determined. The manu- 
facturers' instructions should be followed in every 
case but the following points require special at- 
tention. 

A large part of the wear of plates is due to gas- 
sing so that, while the beginning of a charge may 
be at the 2-hour rate, it' is advisable to keep 
the charging rate slow toward the end of a 
charge; the average charge rate of 8 hours should 
be used whenever possible. 

Cells should be overcharged about once a 
month to get rid of the last traces of sulphate 
and also to even up the cells and make sure 
that they are all charged up to their full capacity. 
Too rapid discharging causes the sulphate to 
form rapidly and tends to cause buckling, this 
. will seldom cause trouble in a well-designed bat- 
tery. Overdischarge however must be avoided, the terminal volt- 
age not being allowed to drop below 1.8 at the normal 8-hour 
rate nor below 1.75 at the 4-hour discharge rate. 

The cell should not be allowed to stand discharged for any 
length of time. If a battery has to stand idle for several months it 
should be charged monthly because even a small leakage current 
will cause enough sulphate to form on the plates to cause trouble 
if it turns into the insoluble form. This monthly charge should 
be continued until there is no further rise in voltage or of specific 




Fig. 172.— Hy- 
dro meter for 
testing the spe- 
cific gravity of 
the electrolyte. 



Art. 186]- STORAGE BATTERIES 157 

gravity and until the cell has been gassing for about 5 hours, one 
may then be reasonably sure that no sulphate has been left on the 
plates. 

Evaporation of the electrolyte should be made good by the 
addition of pure water; the acid does not evaporate and, unless 
there is excessive spraying due to the gases given off, the quantity 
of acid in the cell will not change. 

The specific gravity of the acid used depends on the use to 
which the cell will be put while the permissible change in the 
specific gravity depends on the bulk of acid in the cell and 
should be obtained from the maker. When the cell has to stand 
inactive for long periods, a weak acid is used to lessen the risk of 
sulphation. For automobile work, the quantity of acid in the 
cell should be such that the specific gravity shall not change more 
than from 1.28 to 1.17 between full charge and full discharge; for 
other service a range of from 1.23 to 1.15 is more usual, the gravity 
being measured at a temperature of 70° F. 

Before removing sediment from a cell, the plates should be 
fully charged, then taken out and the separators removed. The 
plates and tank should then be washed with water and the whole 
battery put back into commission before the plates have time to 
dry. 

The gases formed during overcharge are explosive so that a 
naked flame should be kept away from the battery room. The 
room also should be well ventilated and the floor and walls should 
be of some acid-resisting material such as vitrified brick. The 
room should be obscured from direct sunlight, which tends to 
warp the plates; a heating system should be put in if the battery 
has to operate in a cold climate, so as to maintain the capacity 
under all conditions. 

186. Action of the Edison Battery. — If a plate of nickel oxide 
(NiC>2) and one of iron (Fe) are placed in a solution of caustic 
potash (KOH), a battery is formed with the nickel oxide plate 
at the higher potential. 

If current is drawn from this battery, the oxide (Ni02) is re- 
duced to a lower oxide (Ni 2 3 ) while the iron is oxidized to form 
FeO, and the cell is gradually discharged; 

2Ni0 2 + Fe = Ni 2 3 + FeO 

If now current from some external source is forced through 
the cell in the opposite direction so as to cause electrolysis, the 



158 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxiti 

action is completely reversed. The negative ions are attracted 
to the positive Ni20 3 and the higher oxide NiC>2 is reformed while 
the positive H ions are attracted to the negative FeO and reduce it 
to iron 

Ni 2 3 + FeO = 2Ni0 2 + Fe 

the result of charge and discharge is a transfer of oxygen from one 
plate to the other; the strength of the electrolyte is not changed 
so that the quantity required is less than for an equivalent lead 
cell. 

After the battery has been completely charged, the hydrogen 
and oxygen appear as gases which bubble up through the 
electrolyte just as in the lead cell. 

187. Construction of the Plates. — The positive or nickel plate 
shown in Fig. 173 consists of a nickel-plated steel grid carrying 
perforated steel tubes, one of which is shown in diagram B . These 
tubes are heavily nickel plated and are filled with alternate layers 
of nickel hydroxide and flaked metallic nickel. The hydroxide is 
acted on electrochemically and becomes nickel oxide. This oxide is 
such a poor conductor of electricity that the flaked nickel is added 
to bring the inner portions of the oxide into metallic contact with 
the surface of the tubes and thereby reduce the internal resistance 
of the cell. 

Each tube has a lapped spiral seam to allow for expansion, and 
is reenforced with steel rings to prevent the tube from expand- 
ing away from and breaking contact with the enclosed active 
material. 

The negative or iron plate shown in Fig. 173 consists of a nickel 
plated steel grid holding a number of rectangular pockets filled 
with powdered iron oxide. Each pocket is made of two pieces of 
perforated steel ribbon flanged at the side to form a little flat box 
which may be filled from the end. 

188. Construction of an Edison Battery. — A number of like plates 
are connected in parallel to form a group, there being one more 
plate in the negative than in the positive group. Two sets of 
plates are then sandwiched together as shown in Fig. 174, ad- 
joining plates being separated from one another by strips of hard 
rubber. End insulators A are provided with grooves which 
carry the edges of the plates, and thereby act as spacers and at 
the same time insulate the plates from the steel tank. The out- 
side negative plates are insulated from the tank by sheets of hard 



Art. 188] 



STORAGE BATTERIES 



159 



rubber, while the whole unit rests on the rubber rack B by which 
the plates are insulated from the bottom of the tank; this rack is 
shallow since very little space is required for sediment in an 
Edison cell. 




A. Pocket for negative plate. 



B. Tube for the positive plate. 








Positive .plate Negative plate. 

Fig. 173. — Plates of an Edison Battery. 



The tank, which is made of cold rolled steel welded at the joints, 
is corrugated for strength as shown in Fig. 175 and is heavily 
nickel plated as a protection against rust. The cover is of the 



160 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxiii 

same material and is welded to the rest of the tank after the plates 
have been put in place. This cover carries two terminals, as 
well as a combined gas vent and filling aperture A. When the 
cover b is closed, the hemispherical valve a closes the aperture 
and prevents the escape of electrolyte, but allows the gases 
generated on overcharge to escape as soon as the pressure in 
the tank becomes high enough to raise the valve. 

The electrolyte used consists of a 21 per cent, solution of 
potash in distilled water to which a small amount of lithia is 




Fig. 174. — Plate groups of an 
Edison cell. 



Fig. 175.— Top of the jar of an Edi- 
son cell. 



added. No corrosive fumes are given off from this electrolyte 
so that no special care need be taken in mounting the cells. 

189. The Voltage of an Edison Battery. — Fig. 176 shows how 
the voltage of an Edison battery changes when the battery is 
charged and then discharged. The voltage characteristics are 
similar to those of a lead battery. 

There is no lower limit to the voltage of an Edison battery 
because in it there is nothing eqivalent to sulphation, but dis- 
charge is not continued below a useful lower limit. 

190. Characteristics of an Edison Battery. — These batteries 
are rated at a 7-hour charging rate and a 5-hour discharge rate 



Art. 190.] 



STORAGE BATTERIES 



161 



with the same current in each case, the ampere-hour efficiency 
being about 82 per cent, at this rate and the internal heating 
not more than permissible. A higher rate of discharge may be 
used so long as the internal temperature does not exceed about 
45° C; continual operation at higher temperatures shortens 
the life of the cell. A longer charge rate than 7 hours should 
not be used because, with low currents, the iron element is not 
completely reduced; this however does not permanently injure 
the cell but makes it necessary to overcharge the cell at normal 
rate and then discharge it completely to bring it back to normal 
condition. 

Because of the comparatively high internal resistance of the 



tf.U 




































SO 

■2 20 






Lead 


Cell 






o 

































g 
5 
131.0 






Edi 


sonC 


ell 











































12345678 
Hours 

Fig. 176. — Charge and discharge curves of a lead cell and an Edison cell. 



Edison battery, the volt efficiency is lower than in the lead cell, 
as may readily be seen from Fig. 176, and, since the ampere- 
hour efficiency is not any higher, the watt-hour efficiency of the 
Edison cell is also lower. 

The great advantages of the Edison cell are that it is lighter than 
the lead cell and is more robust, it can remain charged or dis- 
charged for any length of time without injury, and so little sedi- 
ment is formed that the makers seal it up. Since no acid fumes 
are given off, the cell may be placed in the same room as other 
machinery without risk of corrosion of that machinery. 

The chief disadvantage of the Edison cell, in addition to its 
high cost, is that its efficiency is lower than that of the lead cell. 



V 



K > 



i 



CHAPTER XXIV 

OPERATION OF GENERATORS 

191. Operation of the Same Shunt Machine as a Generator or 
as a Motor. — The generator G, Fig. 177, driven in the direction 
shown, supplies power to the mains mn. The same machine, 
operating as a motor from mains of the same voltage and polarity, 
is shown at M; the direction and the strength of the shunt field 
are unchanged, the direction of the armature current is reversed, but 



i 





Generator Motor 

Fig. 177. — Operation of the same shunt machine as a generator and as a 

motor. 



the direction of motion, determined by the left-hand rule, page 7 
is the same as in G. Since the back e.m.f. when the machine is 
operating as a motor has to be practically equal to E, the e.m.f. 
of the machine when operating as a generator, the machine must 
run at the same speed in each case. 

In diagram A, Fig. 178, m and n are two mains kept at a 
constant voltage E by the generators in a power house, and D 
is a single shunt generator of the same voltage running at normal 
speed. If the voltage E d is exactly equal to E and the polarity 

162 



Art. 192] 



OPERATION OF GENERATORS 



163 



is as shown, then no current will flow in the lines a and b when 
the switch S is closed. If the excitation of D is now increased 
slightly so that the voltage generated in the machine is greater 
than the line voltage E, then current will flow in the direction 
of the generated voltage, as shown in diagram B, and the machine, 
operating as a generator, will supply power to the circuit mn. 
If now the excitation of D is decreased so^ that the voltage gen- 
erated in the machine is less than the line voltage E, then current 
will flow in the direction of the greater voltage, that is, in a 
direction opposite to that of the generated voltage, as in diagram 
C, and the machine, operating as a motor in the same direction 
as before, will take power from the circuit mn. Thus by merely 
varying the excitation of D it may be made to act as a generator 
or as a motor. 




F 



i ( 




B C 

Fig. 178. — Operation of the same shunt machine as a generator and as a 

motor. 



192. Loading Back Tests. — Load tests on large electrical ma- 
chines must be made by some method whereby 'the power de- 
veloped by the machine is not dissipated but is made available 
for the test, otherwise the power-house capacity may not be 
large enough to allow many machines to be tested, while the cost 
of such tests will be excessive. 

If the machine to be tested is a generator, it is driven at normal 
speed by a motor of the same voltage but of larger capacity, and 
both machines are connected to the power house mains as 
shown in Fig. 179. The motor M is started up by means of a 
starting box in the usual way, and the generator G is excited until 
its voltage is equal to E, the switch Si is then closed and a volt- 
meter V is placed across the switch $2. If the reading of this 
voltmeter is twice normal voltage, then the polarity of G must be 



164 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxiv 



reversed by reversing the shunt field, but if the reading is zero, then 
the switch $2 may be closed and the generator G thereby con- 
nected to the mains. There will then be no current in the leads 
a and b so that the generator will be running light, and the motor 
will be taking from the power house only that power required to 
operate the two machines at no-load. 

If the excitation of G is now increased so as to increase its 
generated voltage, then current will flow in the direction shown; 
the machine will deliver power to the circuit ran while the motor 
M, which drives G, will take from this same circuit an amount of 
power equal to the output of G plus the losses in the two machines, 
of which the portion required to supply the losses is all that is 

taken from the power house since 
the output of G is sent back into the 
power mains. 

193. Parallel Operation.— The 
load on a power station is generally 
distributed among several genera- 
tors connected in parallel with one 
another, so that a breakdown of 
one unit will not seriously cripple 
the station. Parallel operation of 
generators has the additional ad- 
vantage that the number of generating units in operation can be 
changed with the load, so as to maintain the individual machines 
at approximately full-load, at or near which load they operate 
with their highest efficiency. 

194. Shunt Generators in Parallel.-- A and B, Fig. 180, are 
shunt generators which feed into the same mains ra and n. Sup- 
pose that A has been carrying all the load and that it has become 
necessary to connect generator B to the mains to share the load. 
This latter machine is brought up to speed with the switch S open, 
its field rheostat is adjusted until E b is equal to E, and the switch 
S is closed. The load on B is then zero. To make the two 
machines divide the load, the excitation of B is increased so as to 
increase its generated voltage and thereby cause the machine to 
deliver current to the mains. 

If, due to a momentary increase in speed or for some other reason, 
machine A takes more than its proper share of the total load, the 
voltage of A drops since it is a shunt generator, see page 73, and 
part of this load is automatically thrown on B, the machine with 




Fig. 179. 



—Loading back test on 
a generator. 



Art. 195] 



OPERATION OF GENERATORS 



165 



the higher voltage at that instant. Furthermore, if the engine 
connected to B fails for an instant, that machine slows down, its 
generated voltage drops and the load is automatically thrown on 
A ; if this generated voltage drops far enough, then current flows 
from the line to operate machine B as a motor at normal speed and 
in the same direction as before, see 
page 162, but, as soon as the engine 
recovers, this machine again takes 
its share of the load. The opera- 
tion of two shunt machines in paral- 
lel is therefore stable, each machine 
refuses to take more than its proper 
share of the load and yet helps the 
other machine when necessary. 

To disconnect machine B, its ex- 
citation should be reduced until A 
is carrying all the load, the switches 
S may then be opened. 

195. Division of Load among 
Shunt Generators in Parallel. — The external characteristics of 
the two shunt generators are shown in Fig. 181. When the line 
voltage is E, the currents in the machines are I a and I b and the 
line current is I a + h> If the current drawn from the mains 



*m. 





Fig. 



180.- — Parallel operation of 
shunt generators. 




l a H>I, 



! i 

'in I* 



Armature Current 
Fig. 181 

—181, 182 and 183. 



Armature Current 




Fig. 182 Fig. 183 

Figs. — 181, 182 and 183. — Division of load between two shunt generators in 

parallel. 

decreases, the voltage E rises and the currents in the two ma- 
chines are then i a and 4 when the line current is i a + ib. 

To make machine A take a larger portion of the total load, its 
excitation must be raised so as to raise its characteristic as shown 
in Fig. 182. 

If a 100-kw. and a 400-kw. machine have the same regulation 



166 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxiv 

and therefore the same drop in voltage from no-load to full- 
load then, as shown in Fig. 183, the machines will divide the 
load according to their respective capacities. 

196. Compound Generators in Parallel. — A and B, Fig. 184, 
are two compound generators which are operating in parallel. 
If, due to a momentary increase in speed, machine A takes 
more than its proper share of the total load, the series excita- 
tion of A increases, its voltage rises, and it takes still more of 
the load, so that the operation is unstable. 

To prevent this instability, the points e and /, Fig. 185, are 
joined J^y a connection of large cross section and of negligible 
resistance, called an equalizer connection. The series coils P 
and Q are thus connected in parallel with one another between 
the equalizer and the negative main n, as is shown more clearly 





Fig. 184. — Compound generators 
in parallel. 




Fig. 185. — Compound generators 
in parallel, an equalizer being sup- 
plied and the machine A being 
over-loaded. 



in Fig. 186, and the total current from the negative main n 
always passes through these coils in one direction and divides 
up between them inversely as their resistance, independently of 
the distribution of the load between the machines. If now, due 
to a momentary increase in speed, machine A takes more than 
its proper share of the total load, as shown in Fig. 185, and 
therefore less is left for machine B, the series excitation of the 
two machines is unchanged, since the total load is unchanged, 
so that the machines act as shunt generators with a constant 
superimposed excitation and the voltage of A decreases and that 
of B increases, and part of this load is automatically thrown 
on B, the machine with the higher voltage; the operation of the 
machines has therefore been made stable by the addition of the 
equalizer connection. 



Art. 197] 



OPERATION OF GENERATORS 



1G7 



To connect machine B in parallel with machine A which is 
already running, bring the machine up to speed with the switches 
a, b, and c open, close switches b and c in order to excite the series 
coils, then adjust the shunt excitation until Eb is equal to E, 
and finally close switch a, the machine may then be made to take 
its share of the load by increasing its shunt excitation. To 
disconnect the machine, its shunt excitation should be reduced 
until all the load has been transferred to A, the switches should 
then be opened in the reverse order. 

For large machines three separate switches are generally used. 
For smaller machines the- switches b and c are often combined 
to form a double pole switch. When the machines are a con- 
siderable distance from the mains m and n, the equalizer is often 
run straight between the machines as shown in Fig. 186. 




Equalizer L 





Fig. 186. — Compound generators in parallel, showing methods of changing 

the compounding. 



197. Division of Load among Compound Generators. — When a 
single compound generator has too much compounding, a shunt 
in parallel with the series field coils will reduce the current in these 
coils and so reduce the compounding, page 77. 

When one of a number of compound generators in parallel 
is found to take more than its share of the load, then its com- 
pounding must be reduced, this, however, can no longer be ac- 
complished by placing a shunt in parallel with the series coils of 
that machine, for example the shunt S will not only reduce the 
current in the series coils Q but will at the same time reduce the 
current in the series coils P since the two sets of series coils and 
the shunt S are then all connected in parallel between the negative 
main and the equalizer, as shown in diagram X, and the total 
line current will divide among them inversely as their resist- 



168 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxiv 

ance. The compounding of both machines will therefore be 
reduced. 

To reduce the current in the series coils Q without at the same 
time reducing that in the coils P, a resistance must be placed in 
series with Q as shown in diagram Y. 



CHAPTER XXV 

OPERATION OF GENERATORS AND BATTERIES IN 
PARALLEL 

198. Isolated Lighting Plants. — The engine and generator 
capacity in such plants is generally sufficient for the total 
number of lamps connected, but since the lamps are seldom all 
in use at one time, the plant operates at partial load and con- 
sequently with low efficiency. When a suitable storage battery 
is installed, the generator may be operated for a few hours to 
charge the battery and may then be shut down, the battery 
being left connected to supply the load current. 

199. Lighting Plants for Farm Houses. — The equipment for 
such plants is shown diagrammatically in Fig. 187, 30-volt 
tungsten lamps being used since they have stronger filaments 
than 110- volt lamps and can therefore be made in smaller sizes, 
see page 353. 

The voltage of a lead cell varies from about 2.65 volts on full 

charge to 2.2 volts at the beginning of discharge and 1.8 volts 

at the end of discharge, so that if 16 cells are connected in series 

then: the battery voltage on full charge = 16 X 2.65 = 42.5 volts, 

at the beginning of discharge = 16 X 2.2 = 35.2 volts, 

at the end of discharge = 16 X 1.8 = 28.8 volts. 



If the voltage across the lamps is raised much above 35 volts, 
then the 30-volt lamps will be burnt out, so that the lamp circuit 
must be disconnected while the generator is charging the battery. 

Specify the generator and battery for a lighting plant with a connected 
load of 24 tungsten lamps of 15 watts and 12 candle power each, 
watts per lamp =15 

current per lamp = 15/30 = 0.5 amp. 
current for 24 lamps = 0.5 X 24 = 12 amp. 

battery capacity at normal 8-hour rate = 12 X 8 = 96 amp. -hours 
maximum generator voltage = 16 cells at 2.65 volts = 42.5 volts 
charging current at 8- hour rate = 12 amp. 
generator output = 12 X 42.5 = 510 watts 

169 



170 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxv 

If the average daily load on the battery is 6 lamps for 4 hours then the 
ampere-hours taken each day = (6 X 0.5) X 4 = 12 and a 96-amp.-hour 
battery will supply this load for 96/12 or 8 days without having to be re- 
charged. 

To prevent the generator from being connected in parallel 
with the battery except when its voltage is higher than the 
battery voltage, an automatic switch S is supplied. To charge 
the battery, the lights are disconnected, the generator which 
is shunt wound is started up by the engine, and the shunt 
rheostat r is gradually cut out to raise the generator voltage 
until the value is reached for which the solenoid F was set, the , 
pull of this solenoid then closes the switch S and connects the 
generator in parallel with the battery. The generator then 
delivers current to the battery, which current flowing through 
the coil H, adds to the pull of F and helps to keep the switch S 
closed. 



L 




Lamps 



r 



SS 



Carbon Pile 



Fig. 187. — Connection diagram for a small iso- Fig. 188. — Lamp circuit 
lated lighting plant. regulator. 



If, due to a loose field connection or for some other reason, the 
generator voltage drops below that of the battery then current 
flows back through the coil H in such a direction as to oppose the 
pull of F; the switch S is thereby released and the generator dis- 
connected from the circuit. The switch S therefore acts as a 
reverse current circuit breaker. 

200. Lamp Circuit Regulator. — One objection to the above 
system is that the voltage across the lamps varies with the 
battery voltage from 35.2 to 28.8 and the life of the lamps is 
shortened due to the high voltage while the lighting is unsatis- 
factory when the voltage is below 30 volts. This trouble may 
be overcome by the addition of an automatic regulator. A very 
simple type of regulator for this purpose is shown diagram- 
matically in Fig. 188 and consists of a carbon pile resistance 



Art. 202] OPERATION OF GENERATORS AND BATTERIES 171 

R inserted in the lighting circuit, and a shunt solenoid P by which 
the pressure on the carbon pile is varied. If the voltage across 
the lamp circuit increases, the current in the solenoid P increases 
and lifts the lever L, thereby reducing the pressure on the 
carbon pile R and increasing its resistance, so that the voltage 
drop across the carbon pile increases and the lamp voltage re- 
mains approximately constant. A more elaborate regulator 
of this type is described on page 182. 

201. Small Isolated Power Stations. — In such stations, 
provision must be made for charging the battery and also for 
carrying the day load at the same time. This is accomplished 
either by resistance control, end cell control or booster control. 

202. Resistance Control. — To take the case of a 110-volt plant. 
The number of cells in series is 110/1.8 = 60 and, to charge 
them in series, would require a maximum voltage of 60 X 2.65 = 




r^'I'l'- 
T l-I-I- 



-H'l 



.A- Charging 

fH_.li- 



few 



j#4i 



Overload and Reverse 
Current Circuit Breaker 



B- Discharging 



Fig. 189. — Resistance system of battery control. 



160 volts. But since the day load has to be carried by the 
generators while the battery is being charged, it is not permis- 
sible to raise the generator voltage above 110 volts. This 
difficulty is overcome by dividing the battery into two halves 
for charging purposes and connecting them to the generator 
as shown in Fig. 189, the maximum battery voltage during charge 
will then be 80 volts and the remaining part of the 110 volts must 
be used up in the resistance R, by means of which resistance 
the charging current may be regulated. 

When the battery is fully charged the cells are reconnected 
in. series by throwing the switches over into position Si and, 
since the battery voltage at the beginning of discharge is 



172 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxv 

60 X 2.2 or 132 volts while the generator voltage is only 110 volts, 
the same resistance R must be kept in the battery circuit to 
control the discharge. 

203. End Cell Control.— With this system of control, shown 
diagrammatically in Fig.' 190, the battery is charged in series from 
a generator which has a voltage range up to 160 volts, the charg- 
ing current being regulated by the generator field rheostat. Two 
end cell switches C\ and C 2 are required so that the 110-volt load 
may be supplied while the battery is being charged. 

When the battery is at the end of discharge, the number of 



-BSi 



4 — a Si g ^ 

Overload 

and 

i Reverse 

r~ Current 

Cb. 

— 160— > 

Volts 

3 



Over] 
C.br-\ 

R 



110 Volts 

on Load .Circuit. 



-^-iCi 




Load C - End Cell 
Switch 



3 



| 110 Volts ~| 



A - Charging B --Discharging 

Fig. 190. — End cell system of battery control. 



cells for 110 volts is 110/1.8 or 61 while at the beginning of 
discharge 110/2.2 or 50 cells are all that are required, and 11 end 
cells must be so arranged that they can be gradually connected 
in circuit as the battery discharges. When the end cell switches 
are in the position shown in Fig. 190, there are 50 main cells 
and 4 end cells on the lighting circuit while all the cells are being 
charged in series; these 4 cells on the lighting circuit will 
gradually be cut out as the battery becomes more fully charged 
and the voltage rises. 

When the battery is fully charged, the switches are thrown over 



Art. 204] OPERATION OF GENERATORS AND BATTERIES 173 

into the position Si and the generator and battery are thereby 
connected in parallel across the mains. 

In order that the end cell switch will not open the circuit when 
passing from one contact to that adjoining nor yet will it short 
circuit any one cell, this switch is generally constructed as shown 
diagrammatically in Fig. 190 with a main contact a and an aux- 
iliary contact b electrically connected through a resistance r but 
otherwise insulated from one another. As the switch is moved 
over the contacts, it stands for an instant in the position shown 
and bridges one ceil, but the resistance r keeps the current that 
flows through this cell from being dangerously large. 

Since the end cells are gradually put in circuit as discharge 
proceeds, they are never so completely discharged as the rest of 
the battery, so that, when the battery is being recharged with all 
the cells in series, the end cells should be cut out one by one as 
they become fully charged and begin to gas freely. 



sFl 4=^=0=3 J" 



Si 



£*=>s 



yy+ky 



1- 



L0J ! n so vo 

i ft 



A Battery Charging 

A. 



B Battery Discharging 

Fig. 191. — Booster charge and end cell discharge system of battery control. 



With this system of control there is not the resistance loss that 
is present in the resistance control system, but the outfit is more 
costly. 

204. Booster Charge, End Cell Discharge.— For larger self- 
contained plants, the high voltage for charging is obtained by 
connecting an additional generator B, called a booster, so that 
its voltage is added to that of the main generators. To charge 
the battery in Fig. 191, the switches must be thrown over into 
position Si and then, if the generated voltage is 110 and the maxi- 
mum voltage on charge is 160, the booster has to supply 50 volts. 

The booster is generally driven by a constant speed motor and 



174 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxv 



is excited from the generator mains, the field rheostat having 
sufficient resistance to allow the booster voltage to be reduced to 
about 2 volts. 




Fig. 192. — Large end cell switch. 

When the battery is fully charged, the double throw switch 
is thrown over into the position shown in Fig. 191 so as to connect 
the battery directly across the mains, the discharge voltage is 

1800 



1600 



1400 



1200 



»1000 



800 



600 



400 



200 



























M 
















A E 
Dis< 


attery 
harge 












1 
















/ Generator Lo 


id 














^J 














:\ 


Generator Load / 


3atteryC 


harge 






B \ 


JB 


\~^^y^ 















12 3 6 9 A.M. 12 3 6 9 P-^-12 

Noon Midnight 

Fig. 193.— Daily load curve on a small station. ■ 

then regulated by the end cell switch C. The booster may be 
used for this purpose if the discharge current is not too large, see 
next article. 



Art. 205] OPERA TION OF GENERA TORS AND BA TTERIES 1 75 

The end cell switch for such plants, when of large size, gener- 
ally takes the form shown in Fig. 192, a laminated brush being 
moved across a series of contacts by a motor-driven operating 
screw. This motor may be provided with push-button control if 
desired. 

205. Capacity of Battery. — The irregular curve in Fig. 193 is 
the load curve on a small power station, and a battery is required 
to supply all the current over 1000 amp. 

The batteiy discharge = cross hatched area A 

= 1140 amp. hours 
the time of discharge = 2.7 hours 
the average discharge current = 420 amp. 
the maximum discharge current = 800 amp. 

When the discharge takes place at the 2.7-hour rate, the capacity of the 
battery is only 75 per cent, of the normal capacity, see page 153, therefore 
the normal capacity of the above battery = 1140/0.75 

= 1500 amp .-hours. 

To recharge the battery, about 20 per cent, more ampere-hours must be 
put in than were taken out on the previous discharge when this discharge 
was at the 2.7-hour rate, see page 154. 

therefore : 

the charge required = 1140 X 1.20 

= 1370 amp .-hours 
- shaded area B 
the battery charges for 8.7 hours 
the maximum charging current = 230 amp. 

or is at the 1500/230 = 6.5-hour rate 
the average charging current = 1370/8.7 = 157 amp. 

the booster has to carry a maximum of 230 amp. and must be designed for 
a maximum voltage of 50 the output is therefore 50 X 230 = 11.5 kw. 

This booster could not be used to help the battery to discharge 
because the 800-amp. discharge current would burn up a 230- 
amp. booster, end cells must therefore be supplied. 

In working out this problem it has been assumed that the batter}' 
will never be called on to deliver more than 1140 amp. -hours at 
the 2.7-hour rate. 

No attempt has been made to determine whether or not the 
above outfit is the most suitable for the particular service, this 
is a question which can be answered only by trial of different com- 
binations of generating and storage equipment, the total operat- 
ing cost including maintenance and depreciation being figured out 
in each case. 



176 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxv 

206. Batteries for Rapidly Fluctuating Loads. — The load on the 
power house of plants such as rolling mills fluctuates so rapidly 
that hand-operated boosters and end cells cannot follow the 
fluctuations and automatic control becomes necessary. Onh r 
two of the many systems that have been devised will be described, 
the principle in each case being to control the field excitation of 
the booster by means of the load current in such a way that, 
when the load is heavy the booster will assist the battery to 
discharge, and when the load is light the booster will assist the. 
generators to charge the battery. 

207. The Differential Booster in its simplest form is connected 
as shown in Fig. 194 and is supplied with a set of shunt coils A 
and a set of series coils B which coils are connected so that their 
m.m.fs. are in opposition. With normal load on the generator, 



ries Coils Shunt Ceils 




Fig. 194. — Differential booster svstem of batterv control. 



the m.m.f. of the series coils is equal and opposite to that of the 
shunt coils and the resultant magnetic field is zero, so that the 
booster voltage is zero and the battery neither charges nor 
discharges. 

If the load on the generator increases, the series excitation of 
the booster becomes greater than the shunt excitation and the 
booster voltage is then added to the battery voltage and causes 
the battery to discharge and carry the larger part of the excess 
load. If the generator load now becomes less than normal then 
the series excitation decreases and the shunt excitation is now the 
greater so that the booster voltage is reversed, opposes the battery 
voltage, and thereby helps the generator to charge the battery. 

208. Carbon Pile Regulator. — The diagram of connections for 
a booster system controlled by a carbon pile regulator is shown in 
Fig. 195. The booster B is excited by the coil E which takes 



Art. 208] OPERATION OF GENERATORS AND BATTERIES 111 

current from a small exciter, the field excitation of which is con- 
trolled by a regulator consisting of two carbon piles r 1 and r 2 . 
These carbon piles are subjected to pressure by the lever L, to 




Fig. 195. — Carbon pile regulator for the automatic control of a battery. 

one end of which is attached the plunger of the operating so- 
lenoid S and to the other end a spring T, the tension of which may 
be adjusted to counterbalance the pull of the solenoid when any 




Fig. 196.— Carbon pile regulator for a battery. 



desired current is flowing through it. The diagram of con- 
nections of the exciter field coil circuit is shown in diagram A. 
The voltage e x tends to send a current i\ = ei/(r -f r{) through 



12 



178 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxv 

the coil F while the voltage e 2 tends to send a curreut i 2 = 
e 2 /(r-{-r 2 ) through the same coil in the opposite direction. 

If the generator load is normal then the pull of the solenoid S 
is equal to the pull of the spring T and t\ is then equal to r 2 , i\ is 
equal to i 2 , diagram A, and no current flows in the coil F, so that 
the exciter voltage and the booster voltage are both zero. 

If the generator load increases, the pull on r\ increases and its 
resistance decreases and is now less than r 2 so that i\ becomes 
greater than i 2 and current flows in the coil F in the direction of 
i\ and the booster voltage adds to the battery voltage and causes 
the battery to discharge and carry the larger part of the excess, 
load. 



6000 




1 2 3 4 5 6 78 9 10 

Minutes 

Fig. 197. — Load curves on the power house of a steel mill. 



If the generator load now becomes less that normal, the pull on 
r x decreases and its resistance increases and is now greater than 
r 2 and current now flows in the coil F in the direction of i 2 , the 
booster voltage is thereby reversed and adds to the generator 
voltage and thereby helps the generators to recharge the battery. 

The curves in Fig. 197 show how nearly constant the generator 
load may be maintained by such a regulator, while the indicator 
cards in Fig. 198 show the effect of the regulator in maintaining 
the steam consumption constant, an operating condition which is 
favorable to economy in steam consumption. 



Art. 209] OPERATION OF GENERATORS AND BATTERIES 179 

The average load on the generator is that at which the pull of 
the solenoid S is exactly counterbalanced by the tension of the 
spring T, for then the booster field excitation is zero; this load 
may be adjusted by varying the tension of the spring. 

One advantage of the externally controlled booster over the 
differential booster is that the former machine is a standard shunt 
generator whereas the latter has special series field coils and 
heavy cables leading to these coils; some idea of the section of 
copper required for these coils and cables may be obtained from 
Fig. 196 which shows the section of copper required to carry the 
current in a carbon pile regulator. 



A — Battery in circuit. B — Battery out of circuit. 

Fig. 198. — Indicator cards from the steam engines in the power house of a 

steel mill. 

209. Floating Batteries. — If a battery is connected in par- 
allel with the generator and with the load, as shown in Fig. 
199, then the condition to be fulfilled before the battery 
will carry the peak load is that the voltage regulation of the 
generator shall be worse than that of the battery, so that, as the 
load increases, the generator voltage drops below that of the 
battery and the battery has to discharge, while if the load 
decreases, the generator voltage rises above that of the battery 
and the battery is charged. 

Such poor regulation is not desired in a power house, but is 
often found at the end of a transmission line. If the length 
L of this line is considerable and the section of the wire is small, 
then a heavy load on the line will cause the voltage to drop suf- 
ficiently to allow the battery to discharge, while with a light 
load on the line the drop is small and the voltage is then high 
enough to recharge the battery. 

The exact equivalent of a line with poor regulation is shown 
in Fig. 200, which represents diagrammatically a hotel power 
plant supplying lamps L and elevator and other motors M. 



180 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxv 

The generator is flat compounded to maintain the voltage con- 
stant across the lamps, while a battery is inserted to carry the 
peak loads produced by the starting of the elevators. In order 
that the battery may operate properly, there must be a con- 
siderable drop in voltage between the generator and the battery. 
This is arranged for by placing a series wound booster in the 



To Load Generator 



E 2 T To Motors M 



} — tomD- 



To Lighting 
Circuits L 

Fig. 199. — Battery floating at the Fig. 200. — Series booster system of 

end of a line. battery control. 

circuit, the booster being driven at constant speed with its 
voltage opposing that of the generators, so that if the elevator 
motors take a large current the strength of the booster series 
field is increased, this causes the booster voltage to increase 
and the voltage E 2 to drop and thereby allows the battery to 
discharge. 



CHAPTER XXVI 
CAR LIGHTING AND VARIABLE SPEED GENERATORS 

210. The essential condition to be satisfied by any system 
of electric lighting for vehicles is that the voltage across the 
lamps shall be approximately constant for all speeds of the 
vehicle from zero up to the maximum value. 

For train lighting on steam railroads the three methods at 
present in use are: 

a. Lighting from storage batteries, called the straight storage 
system. 

b. Lighting from a constant voltage generator placed on the 
locomotive or in the baggage car, called the head and end system. 

c. Lighting from generators which are belted to the car axle. 
For motor-car lighting, power is supplied by a generator which 

is driven by the engine. 

211. Straight Storage for Trains. — The power for the lighting 
load in this case is supplied entirely by storage batteries which 
are carried under the cars. At the end of each run the batteries 
must be recharged or else replaced by fully charged batteries. 

212. Head and End System. — The lighting load in this case 
is supplied by a 100-volt compound wound generator driven at 
constant speed by a turbine which takes steam from the loco- 
motive. The generating unit may be mounted on the locomo- 
tive or in the baggage car. 

In order that the lights on the train may not go out if the 
train is parted or if the locomotive is disconnected, storage 
batteries must be placed on at least the front and the rear cars 
the general practice being to place a battery on each car. The 
system is then suitable for long runs where cars are not parted and 
where no interchange of equipment is made with other roads. 

The standard battery equipment for such service, if a lead 
battery is used, consists of 32 cells in series so that 

the battery voltage on full charge = 32 X 2.65 = 85 volts 

at the beginning of discharge = 32 X 2.2 = 70 volts 

at the end of discharge = 32 X 1.8 =57 volts 

181 



182 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxvi 

Since 60-volt lamps are used, an automatic regulator must be 
placed in the lamp circuit of each car so as to maintain the lamp 
voltage approximately constant. 

213. Carbon Pile Lamp Regulator. — One t}^pe of regulator 
which depends on the variable resistance of a carbon pile for its 
operation is shown on the middle panel of Fig. 202 and is also 
shown diagrammatically in Fig. 201. 

If the battery is being charged, then the voltage Eb increases 
and the lamp voltage E t also increases slightly. This increases 
the pull on the plunger a which increases the pressure on the 
carbon pile r and decreases its resistance, more current therefore 
flows in the coil b which is connected in series with r across the 
lamp circuit, so that the plunger b is raised and the pressure on the 
carbon pile R is decreased and its resistance increased, the 




Fig. 201. — Lamp circuit regulator. 

greater part of the increased battery voltage is therefore ab- 
sorbed by the resistance R and the lamp voltage remains ap- 
proximately constant. Due to the multiplying effect of the aux- 
iliary solenoid a and carbon pile r, a slight change in the voltage 
across the lamps produces a considerable change in the pull of the 
solenoid b. 

214. The Axle Generator Systems. — With these systems, the 
lighting equipment of each car is self contained and consists of a 
generator driven from the car axle and a storage battery to supply 
power when the car is at standstill. 

An automatic switch must be provided so that the generator 
shall be connected to the battery only when the generator voltage 
is higher than the battery voltage and shall be disconnected 
when its voltage is lower. The generator also must be so con- 
trolled that the charging current shall not be excessive and 



Art. 216] 



CAR LIGHTING 



183 



shall moreover be reduced automatically as soon as the battery 
is fully charged. The combined automatic switch and generator 
regulator shown on the bottom panel of Fig. 202 and diagram- 
matically in Fig. 203 is typical of the 
several carbon pile regulating systems 
on the market. 

215. Automatic Switch. — As the car 
speeds up, the generator voltage increases 
and when it reaches a value which is 
greater than the maximum battery volt- 
age the pull on the plunger P due to the 
current in F becomes large enough to 
close the switch S and connect the gen- 
erator and the battery in parallel. The 
generator then delivers current to the 
lamps and to the battery which current, 
flowing through the coil H, adds to the 
pull on the plunger and helps to keep 
the switch closed. 

If the car now slows down, the speed 
finally reaches a value below which the 
generator voltage is less than that of 
the battery, and current flows back 
through the coil H in such a direction 
as to oppose the pull of F, so that the 
switch S is released and the generator 
disconnected from the circuit, the bat- 
tery being left connected to supply 
power to the lamps. 

216. Generator Regulator. — As the 
speed of the generator increases, the 
voltage of the generator and the charging 
current in the battery both increase. 
This charging current is limited by the 
regulator shown diagrammatically in Fig. 203 which consists of 
a carbon pile rheostat R in the field coil circuit and a solenoid 
B in the battery circuit. The weight of the plunger of B keeps 
the carbon pile compressed, while the upward pull of the sole- 
noid decreases the pressure and thereby inserts resistance in the 
shunt coil circuit and cuts down the voltage of the machine. 
The regulator is adjusted so that the plunger is pulled up and 




Fig. 202.— Control panel 
for a train lighting system. 



184 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxvi 

the pressure on the carbon pile is relieved as soon as the current 
reaches the safe maximum charging rate of the battery. 

If this were the only provision made for regulating the 
generator, then the battery current would be maintained even 
after the battery was fully charged, this would cause excessive 
gassing of the battery and a loss of water from the electrolyte; 
the generator voltage would then be 32 X 2.65 = 85 volts with a 
32-cell battery. 

In order to decrease the charging current once the battery is 
fully charged, an additional relay is provided which limits the 
generator voltage to some value lower than 85 volts. This is 
accomplished by the shunt solenoid A which is connected across 
the generator terminals as shown in Fig. 203. The weight of 




Fig. 203. — Automatic switch, and generator regulator. 

the plunger of this solenoid keeps the carbon pile compressed, 
but when the generator voltage reaches a value somewhat lower 
than 85 volts the plunger is pulled up and the pressure on the 
carbon pile is relieved so that the voltage cannot increase further. 
217. Pole Changer. — The polarity of the leads a and b, Fig. 
203, must be independent of the direction of motion of the car 
in order that the battery-charging current may flow in the 
proper direction, and the generator shunt field coils are connected 
across ab instead of across the generator terminals in order that 
the magnetic field produced may always be in the same direction 
as that due to residual magnetism and may therefore build up 
properly, see page 71. If then the direction of motion of the 
car reverses, the polarity of the generator will also reverse, so that 



Art. 218] CAR LIGHTING 185 

provision must be made for reversing the connections between 
the brushes and the leads a and b. This may be accomplished 
by a double throw switch such as that shown at T, Fig. 203, 
which is thrown over by a mechanism on the generator shaft 
whenever the direction of rotation of the generator is reversed. 

218. The Stone Generator. — The voltage and current of this 
generator are controlled by the slipping of the driving belt. The 
generator is suspended by an adjustable linkL ; Fig. 204, and is 
therefore free to swing toward or away from the driving pulley 
on the axle. The belt is then adjusted to pull the generator out 
of the position in which it would naturally hang and the tension 
put on the belt by this means may be so adjusted by the link L 
that the belt will slip when the load exceeds a certain value. 

The combined automatic switch and pole changer is at the 
commutator end of the machine, see Fig. 205. The contacts B 




Fig. 204. — Suspension of a slipping -belt t} r pe of generator. 

are fixed while the contacts A are carried on a rocker arm C which 
is loose on the shaft and is carried around by friction in the direc- 
tion of rotation until arrested in the position shown by a stop, 
the blade ^4.1 is then opposite B ± and A 2 opposite B 2 . If the rota- 
tion had been in the opposite direction, the arm C would have 
been carried around in this direction until A\ was opposite B 2 
and At opposite B x when the motion of the arm would have been 
arrested by another stop. 

When the speed and the voltage of the generator reach such a 
value that the generator is able to charge the battery, then the 
weights w are thrown out and the arm C with the* switch blades 
attached is pushed along so as to connect the generator and the 
battery in parallel. If the speed decreases to such a value that 
the generator is no longer able to charge the battery, then the 
spring S pulls the switch blades out of contact and disconnects 
the generator. 

The various operations take place in the following order. When 



186 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxvi 

the car starts up, the rocker arm C moves around to its proper 
position, and when the speed becomes high enough, the switches 
AB are closed and the generator charges the battery. As the 
speed increases, the generator voltage and the charging current 
both increase until the load becomes large enough to cause the 
belt to slip, the speed of the generator will then increase no 
further nor will the charging current in the battery increase. 
This system has given satisfaction although its efficiency is not 
so high as that of some of the other systems because of the loss 
in the slipping belt, it also has the objectionable feature that the 
charging current is not reduced after the battery is fully charged, 




Fig. 205. — The Stone train lighting generator. 



219. Lighting Generators for Motor Cars. — The equipment for 
motor-car lighting is similar to that supplied for train lighting 
except that a pole changer is not required since the generator, 
which is driven by the engine, always rotates in one direction 
when the speed is such that the generator is connected in parallel 
with the battery. 

220. Constant Speed Generators. — Several machines operate 
on the same principle as the Stone train lighting generator, the 
slipping belt being replaced by a slipping clutch in order to make 
the outfit as compact as possible. This clutch slips when the 
generator output reaches a predermined value. 

221. Bucking Field Coils. — Differentially compounded genera- 
tors have been successfully used for vehicle lighting, the field 
windings being connected as shown in Fig. 206 where A is a shunt 
winding which is connected across the battery and therefore 



Art. 222] 



CAR LIGHTING 



187 



gives approximately constant excitation while B is a series wind- 
ing which acts in opposition to or bucks the shunt winding. 

When the speed of the generator reaches such a value that 
the generator is able to charge the battery, the automatic switch 
closes. As the speed increases further, the generator voltage 
and the charging current both increase, but this charging cur- 
rent passes through the coils B 
and reduces the excitation of the 
machine, so that the charging 
current is limited. The charging 
current can never exceed the 
value at which the ampere-turns 
of winding B is equal to the am- 
pere-turns of winding A for then 
the flux in the machine and the 
generated voltage would both be 
zero. 

222. Vibrating Contact Regulator. — A compact and efficient 
regulator for a variable speed generator is shown diagrammatically 
in Fig. 207, the car being at standstill. As the car speeds up, 
the generator voltage increases and, when it reaches a predeter- 
mined value, the pull of the magnet M due to the shunt coil a 
closes the main switch S and connects the battery and the generator 




Automatic 
Switch 



Fig. 206.- 



Generator with bucking- 
field coils. 





Fig. 207. — Vibrating contact regulator for a variable speed generator. 

in parallel. The generator then delivers current to the battery 
and to the lamps which current, flowing through the coil b, adds 
to the pull of the magnet and helps to keep the switch S closed. 
As the speed of the generator increases, the voltage of the 
generator and the charging current in the battery both increase. 
This current flows through the coil c and, when it becomes equal 



188 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxvi 

to the safe charging current of the battery, the pull of the magnet 
N closes the contact T and short circuits the generator field coils 
thereby reducing the excitation and limiting the voltage of the 
machine, the battery is therefore protected from excessive charg- 
ing currents. 

In order to decrease the charging current once the battery is 
fully charged, an additional coil d is placed on the magnet N and 
is connected across the generator terminals. The voltage across 
coil d increases as the battery charges until finally the current in 
coil d is able to close the contact T, the voltage can then increase 
no further so that the charging current gradually decreases and 
is comparatively small when the battery is fully charged. 

For motor-car work, a lamp circuit regulator is considered to 
be an unwarranted complication since the battery voltage changes 
slowly and the lamp voltage may be adjusted by means of a 
hand operated rheostat if desired. 

223. The Rosenberg Generator. — Diagram A, Fig. 208, shows 
a generator with the field coils excited from a battery so as to 
produce a flux </>. When the armature revolves in the direction 
of the arrow, e.m.fs. are induced in the conductors, the directions 
of which are shown by crosses and dots, and the voltage between 
the brushes BB is E h while that between the brushes bb is 
zero since the e.m.fs. in the conductors from 6i to a are opposed 
by those in the conductors from a to b 2 . If the brushes BB are 
joined, then current will flow through the armature and will 
produce an armature cross field 0i a , see page 67. 

Consider now the effect of this cross field <j> la acting alone. 
.Since this field is stationary in space it can be represented by 
poles N 2 S 2 as in diagram B, and the lines of force 4>-ia are cut 
by the armature conductors and e.m.fs. are induced in the 
directions shown. The voltage between the brushes bb is E 2 
while that between the brushes BB is zero. If the brushes bb 
are joined, then current will flow through the armature and will 
produce an armature cross field 4> 2a . 

In one form of the actual machine shown in diagram C, the 
poles NiSi are excited from the battery while N 2 S 2 have no field 
coils but carry the cross flux <f>i a . The brushes BB are short 
circuited while the brushes bb are connected to the load. 

As the speed of the generator increases then, referring to dia- 
gram A, the voltage E\ increases causing the current Ii and the 
flux (f> la to increase; referring now to diagram B, the voltage 



Art. 223] 



CAR LIGHTING 



189 



E 2 increases with the flux 4> la and causes I 2 and <t> 2a to increase. 
But 4>2a opposes 4> and demagnetizes the machine, and the 
greater the speed the greater the demagnetizing effect, so that 
I 2 increases by a smaller and smaller amount and over a wide 
range of speed remains practically constant. The current I 2 
can never exceed the value with which 4> 2a is equal to <j> because 
then the flux in the machine would be zero. 

Such a machine is therefore a constant current generator, 




Generator with Alain 
A Field <p Acting Alone 





I0ia 

Generator with Cross Field 
Acting Alone 



C Complete Machine. 

Fig. 208. — Rosenberg train lighting generator. 

and by adjusting the exciting current //, the current I 2 can be 
limited to the normal charging current of the battery; the charg- 
ing current however does not decrease as the battery becomes 
fully charged, a disadvantage this generator possesses in common 
with the slipping belt type of generator. 

The operation of this generator is described in detail because 
it has several interesting applications. It is used for train lighting, 
when it is called the Rosenberg generator; a similar machine called 
the C. A. V. generator is used for motor cars. The machine Has 



190 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxvi 

also been used as a constant current generator for the operation of 
the arc of large searchlights and also for arc welding, since it is 
practically fool-proof and can be short circuited across the operat- 
ing brushes bb without the current exceeding the normal value. 



CHAPTER XXVII 

ALTERNATING VOLTAGES AND CURRENTS 

224. The Simple Alternator.— If the coil abed, Fig. 209, be 
rotated between the poles N and S so that the conductors ab 
and cd cut lines of force, an alternating e.m.f. will be found be- 
tween/ and g, the ends of the coil. The direction of the e.m.f. in 
each conductor, found by the right-hand rule (page 9), is shown 
in diagrams A, B, C and D for different positions of the conductors 
relative to the poles. In diagram A the conductors are not cut- 




Fig. 209. — Simple two-pole alternator. 



ting lines of force, and the e.m.f. between / and g is zero. In 
diagram B the e.m.f s. in the conductors are in such a direction 
as to force current from/ to g through the external circuit, there- 
fore / is the positive and g the negative terminal of the machine. 
In' diagram C the e.m.f. between / and g is again zero. In dia- 

191 



192 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxvii 

gram D the e.m.fs. in the conductors are in such a direction as to 
force the current from gto f through the external circuit, therefore 
g is now the positive and / the negative terminal. The current In 
the external circuit connecting / and g therefore alternates; that 
is, electricity oscillates backward and forward in the circuit. 

If the e.m.f. between / and g is plotted against time then a 
curve such as that in Fig. 209 is obtained. 




Fig. 210. — Two-pole revolving-field alternator. 

Alternators are generally made with stationary conductors 
and a revolving field, as shown diagrammatically in Fig. 210. 
The direction of the e.m.f. in each conductor, shown at different 
instants in diagrams A, B, C and D, may be found by the right- 
hand rule; 1 it must be noted that in the case of a revolving-field 
machine the thumb is pointed in a direction opposite to that of 
the motion of the poles since, according to the rule, it must be 



1 Thumb — direction of motion of conductor relative to magnetic field. 
Forefinger — direction of lines of force. 
Middle finger — direction of e.m.f. 



Art. 226] ALTERNATING VOLTAGES AND CURRENTS 



193 



pointed in the direction of motion of the conductors relative to 
the poles. 

225. The Wave Form. — If the air-gap clearance under the pole 
is uniform in thickness then the lines of force crossing the air 
gap are spaced as shown in Fig. 211, and the e.m.f. in a conductor, 
being proportional to the rate of cutting of lines of force, varies 
as in curve A. By shaping the pole face, however, as in Fig. 212, 
the flux density in the air gap and therefore the rate of cutting of 
lines of force may be so regulated that the e.m.f. in a conductor 
shall vary according to a sine law as shown in curve B. The e.m.f. 
is then said to be simple harmonic and may be represented by the 
formula e = E m sin 6. 





Fig. 211. Fig. 212. 

Figs. 211 and 212. — Wave form of electromotive force. 

226. The Oscillograph. — The shape or form of the e.m.f. wave 
of an alternator may readily be determined by means of an instru- 
ment called an oscillograph, the essential parts of which are shown 
in Fig. 213. 

In the narrow gap between the poles NS of a magnet are 
stretched two parallel conductors ss formed by bending a strip 
of phosphor bronze back on itself over an ivory pulley P. A 
spiral spring attached to this pulley serves to keep a uniform 
tension on the strips, and a guide piece L limits the length of the 
vibrating portion to the part actually in the magnetic field. A 
small mirror M bridges across the strips as shown. 

If current is passed through the strips ss then one strip will 

13 



194 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxvii 

advance and the other will recede and the mirror will thereby 
be tilted about a vertical axis. If the current is alternating then 
the mirror will tilt backward and forward with a frequency equal 
to that of the current, and the deflection will be proportional to 
the current. (The natural frequency of vibration of the mirror 
is at least fifty times the frequency of the current.) 

If now a beam of light is directed on the mirror, the reflected 
beam will move to and fro in the horizontal plane, its displace- 





Fig. 213.— The oscillograph. 




Fig. 214. — Six-pole alternator. 



ment from the zero position c being proportional to the current 
flowing, so that if a photographic film / is moved downward at a 
constant speed a curve will be traced on it by the beam of light, 
which curve will be the wave of the e.m.f. applied at the oscillo- 
graph terminals. 

227. Frequency. — In the two-pole machine shown in Fig. 209, 
the e.m.f. between the terminals passes through a complete cycle 
while the machine makes one revolution. In the six-pole machine 



Art. 228] ALTERNATING VOLTAGES AND CURRENTS 



195 



shown in Fig. 214, the e.m.f. in any conductor a passes through 
three c}^cles, one cycle per pair of poles, while the machine makes 
one revolution. 

If, in an alternator, p is the number of poles then 

the cycles per revolution = « 

t) t 1) in 
and the cycles per sec, called the frequency, = 9 X V ft - 

_p X r.p.m. 

~ 120 _ 
and is represented by the symbol /. 

The frequencies generally found in practice in America are 
25 and 60 cycles per sec, while in Europe 25 and 50 cycles per 
sec are more common. 

A 60-cycle alternator has 24 poles, at what r.p.m. must it be run? 

/ = V X r.p.m/120 
therefore 60 = 24 X r.p.m/120 
and r.p.m = 300. 

The following table gives the relation between poles, speed and 
frequency: 



Revolutions per minute 


Poles 


25 cycles 


50 cycles 


60 cvcles 


2 


1500 


3000 


3600 


4 


750 


1500 


1800 


6 


500 


1000 


1200 


8 


375 


750 


900 


V 


3000/p 


6000/p 


7200/p 



It is important to note that an alternator has a definite speed 
for a given frequency and cannot be run above or below that 
speed without changing the frequency. In a direct-current 
generator, the voltage may be varied by varying the speed, but 
in the case of an alternator this cannot be done without at the 
same time changing the frequency. 

228. Vibrating Reed Type of Frequency Meter. — In this type 
of instrument a number of steel strips are fastened at one end as 
shown in Fig. 215, while the current whose frequency is to be 
determined is passed through the coil A . The reeds are attracted 
twice in a cycle by the electromagnet B and that reed which has 
a natural frequency equal to twice the frequency of the current 



196 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxvii 



Tibrating Strips 




A — Interior construction. 




nil 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

85 90 95 100 105 




B — Scale when the frequency is 100 cycles. C — External appearance. 
Fig. 215. — Vibrating reed type of frequency meter. 




Fig. 216. — Average value of an alternating electromotive force 



Art. 230] ALTERNATING VOLTAGES AND CURRENTS 



197 



will be set in violent vibration. The reeds have their free ends 
whitened and appear as white bands when vibrating. The ex- 
ternal appearance of such an instrument is shown in diagram C. 
229. Average Value of Current and Voltage. — The average 
value of an alternating current or voltage is zero because similar 
sets of positive and negative values occur. The term average 
is generally applied to the average value during the positive part 
of a cycle as indicated in Fig. 216. 



Eav, the average e.m.f. = - Em 1 



lav, the average current = - I n 

IT 



230. The Heating Effect of an Alternating Current.— If a 
direct current / is forced through a resistance of R ohms then the 
power transformed into heat = PR watts. 



Average _„ 
Value of i 




•Fig. 217. 



If an alternating current i = I m sin 6 is forced through the 
same resistance,-then the power transformed into heat at any in- 



l E av X T = 






E m sin d dd 



therefore 



2E T 

2 „ 



198 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxvii 

stant = i 2 R watts and the average power transformed into heat 

= the average value of i 2 R, see Fig. 217. 
= the average value of (I m sin 0) 2 R 
= Im 2 R (average value of sin 2 0) 1 

Irr^R 

2 

= PeffR 

where I e fj = Im/^2 is called the effective current. 

When an alternating current or voltage is specified it is always 
the effective value that is meant unless there is a definite state- 
ment to the contrary, thus an alternating current of 10 amp. is 
one which has the same heating effect as 10 amp. direct cur- 
rent and has a maximum value of 10 V2 or 14.1 amp. and an 

2 

average value of X 14.1 or 9 amp. 

IT 

231. Symbols. — Hereafter in the text the following symbols 
shall be used for alternating voltages and currents : 

sin 0; the instantaneous value 
the maximum value 



e = 


■■ E m sin 6 ; 


i 


= 


Im& 


rii 


E m 






J-m 








E av 


= 


- E m 
IT 

Em 


J- av 


= 


2 

IT 
I, 





; the average value 

E = —j= I = -j- ; the effective value 

232. Voltmeters and Ammeters for Alternating -current Cir- 
cuits. — The moving coil permanent magnet type of instrument 
as used for direct-current circuits was described on page 8. 
If this type of meter was connected into an alternating- current 
circuit the moving coil would be acted on by forces tending to 
turn it first in one direction and then in the other but, due to 
its inertia, the coil itself would not move and the reading would 
be zero. 



1 The average value of sin 2 6 X x = | sin 2 dd 

IT 

| (cos 26 - 1) dd 



1 
2 

( | sin 26 - 6] 



I 



therefore the average value of sin 2 6 = 1/2. 



Art. 232] ALTERNATING VOLTAGES AND CURRENTS 



199 



In order that a moving coil instrument may be used for the 
measurement of alternating currents, the magnetic field and the 
current in the moving coil must alternate together. This result 
is obtained by replacing the permanent magnet by an electro- 
magnet as shown in Fig. 218. The current to be measured is 
passed through the stationary coils A and through the moving 
coil C in series, and the sides of the moving coil are then acted on 
by forces which turn the coil against the tension of the spring S. 





Fig. 218. — Electrodynamometer type of instrument. 

These forces are proportional to the current i in the coil C 
and to the flux produced by the current i in the coils A ; the 
forces are therefore proportional to i 2 and the average turning 
force on the coil C while the current alternates is proportional to 
the average value of i 2 or to the effective current. 

Such an instrument may be used to measure both direct and 
alternating currents and the reading would be the same for 10 
amp. direct current as for 10 amp. effective alternating current. 



CHAPTER XXVIII 



REPRESENTATION OF ALTERNATING CURRENTS AND 

VOLTAGES 

233. Part of a rotating field alternator is shown diagram- 
matically in Fig. 219. As the field rotates, stationary conductors 




Tig. 219. 



"Fig. 220. 



such as a are cut by lines of force and the e.m.f. in these con- 
ductors varies as shown in Fig. 220 and goes through one cycle 
for every pair of poles that pass. 




A B 

Fig. 221. — Representation of an alternating voltage. 

234. Electrical Degrees. — If the vector op rotate in the coun- 
ter-clock direction and the angle 9 is measured from the a;- axis 
then om, the projection of op on the y-axis = op sin and its value, 
plotted in diagram B, passes through one cycle while changes 
through 360 degrees. If now op is drawn to scale equal to 

200 



Art. 235] ALTERNATING CURRENTS AND VOLTAGES 201 

E m , Fig. 220, then om = E m sin B and therefore represents the 
instantaneous e.m.f. e, the curves in Figs. 220 and 221 will 
therefore be alike in every respect if the angle the machine 
moves through in generating one cycle of e.m.f. is called 360 
electrical degrees; in the machine in Fig. 219 this angle is that 
between two consecutive like poles. From this it follows that 
a curve such as that in Fig. 221 may be used to represent the 
voltage generated by an alternator with any number of poles and 
is the curve that would be obtained by an oscillograph. 

235. Vector Representation of Alternating Voltages and Cur- 
rents. — It is generally assumed that these quantities vary accord- 



Fig. 222. — Representation of an alternating voltage and current. 

ing to a sine law and can therefore be represented by sine curves 
as shown in Fig. 222, where 

*, the current at any instant = I m sin 
e, the voltage at any instant = E m sin 

For much of the work on alternating- current circuits and 
machines it is more convenient to represent alternating voltages 
and currents by the corresponding vectors / and E, Fig. 222, from 
which vectors the sine curves may be obtained when desired by 
plotting the vertical components i and e against the angle 0, the 
vectors being rotated in the counter-clock direction. 

If two oscillographs are used as in Fig. 223 one of which, A, 
gives the voltage curve while the other, B, gives the current curve, 
it will be found that the current and voltage do not necessarily 
reach their maximum values at the same instant but that curves 
such as those in diagrams A, B and C, Fig. 223, may be obtained, 
depending on the kind of load connected to the circuit. The 
reasons for the displacement of the current relative to the voltage 
are taken up in Chapters 29 and 30; it is necessary however to 
take up at this point the method of representing such curves by 
vectors. 



202 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxviii 

If two vectors such as E and /, Fig. 224, be rotated in the 
counter-clock direction at the same rate, the angle a between them 
will remain unchanged and the vertical components e = E m sin 
and i = I m sin 6, which are shown at three instants a, b and c, 



qA 



U 



tiB 



V 




A Current Lags Voltage by 
by a Degrees 




B Current in Phase 
with Voltage 

r~\E 



\% 




C Current Leads Voltage 
by a Degrees 

Fig. 223. — Phase relation between current and voltage. 

when plotted against the angle through which the vectors have 
moved measured from any base line ox, will give the sine curves 
of E and i" which curves represent a voltage and a current of the 
same frequency, the voltage E reaching its maximum value a 
degrees before the current becomes a maximum. 



Art. 236] ALTERNATING CURRENTS AND VOLTAGES 



203 



When the current and voltage reach their maximum values at 
the same instant they are said to be in phase with one another. 
When they reach their maximum values at different instants 
they are out of phase and the current is said to be leading or lag- 




o x 



Fig. 224. — Representation of a lagging current. 



) i 


E 




Fig. 227. — The sum of two alternating voltages of the same frequency. 

ging according as it becomes a maximum before or after the vol- 
tage has reached its maximum value. 

236. The Sum of Two Alternating Voltages of the Same 
Frequency. — If two direct- current generators are connected in 



204 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxviii 

series as in Fig. 225, the resultant voltage E 3 is the numerical 
sum of Ei and E 2 the voltages of the two machines. 

If two alternators are connected in series as in Fig. 226, the 
voltage ez at any instant is the numerical sum of ei and e 2 the 
voltages of the two machines at that instant. In the particular 
case shown, the voltage of the second machine lags, or reaches 
its maximum value later than that of the first machine by the 
time required for the poles to pass through a degrees, and the 
curves representing the voltages of the two machines are ei and 
e 2 , Fig. 227. The points on the resultant curve e 3 are obtained 
by adding together the values of e x and e 2 at different instants. 

The voltages of the two machines may be represented by 
the vectors E\ and E 2 drawn to scale with an angle a between 
them because, if these two vectors are rotated together in the 
counterclock direction with this fixed angle a between them, then 
the vertical components e x and e 2 , when plotted against the 
angle turned through by the vectors, will give the curves ei 
and e 2 in their proper phase relation. 

The resultant of these two electromotive forces is the vector 
sum obtained by the parallelogram law and is E3 which repre- 
sents the resultant voltage both in magnitude and in phase 
relation. 



CHAPTER XXIX 



INDUCTIVE CIRCUITS 



237. Inductance. — It has been shown that whenever there is a 
change in the current flowing through a circuit, an e.m.f. of self 
induction is induced which opposes the change of the current, 
see page 11. 

In Fig. 228, when the switch k is closed a current begins to 
flow in the coil and as this current increases in value the flux. 



+ * — 3=3 

1 3 

e J 

C , 

e . 

t , 





Fig. 228. — Growth and decay of current in an inductive circuit. 

threading the coil also increases. Due to the change in the flux, 
an e.m.f. of self induction is induced in the coil which, accord- 
ing to Lenz's law, page 10, acts in such a direction as to oppose 
the increase of the current. 

If, after the current has reached its final value, the switch k 
is suddenly opened, the current in the coil decreases, the flux 
threading the coil also decreases and causes an e.m.f. of self 
induction to be induced in such a direction as to oppose the de- 
crease of the current. This e.m.f. is generally large enough to 
maintain the current between the switch contacts for a short 

205 



206 -PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxix 

interval as they are opened and accounts for much of the flash- 
ing that is seen when a switch is opened in a circuit carrying cur- 
rent. The growth and decay of current in such a circuit is shown 
by the curves in Fig. 228. 

That property of an electric circuit whereby it opposes a change 
in the current flowing is called the self induction or the inductance 
of the circuit; the two terms have the same meaning but the 
term inductance is generally used in engineering work. 

238. Make and Break Spark Ignition. — One method of igniting 
the gas in a gas engine cylinder is based on the above properties of 
the inductive circuit; the essential parts of the mechanism are 
shown in Fig. 229. 




iMGr (!) 




End view. 



Section through cylinder. 



Fig. 229. — Make and break method of gas ignition. 



When the contact at x is closed, current flows in the direction 
of the arrow. When the cam c has reached a predetermined 
position, the spring s opens the contact x and the current is main- 
tained across the gap by the inductance coil L. 

The current in the circuit changes as shown in Fig. 228 so that 
the contact x must be closed long enough to allow the current to 
grow to its full value, but should not be closed too long or the 
batteries will run down. 

239. The Coefficient of Self Induction of a circuit is defined 

as the flux interlinkages per unit current. If, in Fig. 228, a 

current I produces a flux <j> which links the n turns of the coil 

then the flux interlinkages = ncf> and the flux interlinkages per 

n4> 
unit current = -*-• 



Art. 240] INDUCTIVE CIRCUITS 207 

A larger unit is used in practice and L, the coefficient of self 

induction in henries = -y-10 -8 where I is in amperes. 

If the current I is changing, the flux </> is also changing and a, 
voltage of self induction is induced in the coil which is equal to 

e S i = — n-rrl0~ 8 volts, see page 9 1 

and since L = -y-TO -8 

therefore Ldi = nd4> 10 -8 

and e si = — L ,, volts 

240. Alternating Currents in Inductive Circuits. — If an al- 
ternating current is flowing in an inductive circuit then, since the 
current is always changing, there must be an induced e.m.f. of 
self induction opposing the change. If the current is represented 

Ea 




Fig. 230. — Voltage and current relations in an inductive circuit, 
by curve /, Fig. 230, then between a and b, during which interval 
of time the current is decreasing, the e.m.f. of self induction, to 
oppose this decrease, must be positive, while between b and c, dur- 
ing which interval of time the current is increasing, the e.m.f. of 
self induction, to oppose this increase, must be negative. At the 
instants a, b and c the current is not changing and at these in- 
stants the e.m.f. of self induction must be zero. The e.m.f. which 
satisfies all those conditions is represented by the curve E si , 2 
Fig. 230. 

In order to force an alternating current I through a circuit, the 
applied e.m.f. must be large enough to overcome the e.m.f. of 
self induction and also the resistance of the circuit and, in the 

1 The minus sign is used because the e.m.f. opposes the change of the 
flux. 

2 It is important to note that the generated voltage E S i lags the current 
i and therefore the flux by 90 degrees. 



208 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxix 

extreme case of an inductive circuit of negligible resistance, the 
applied e.m.f. must be equal and opposite to the e.m.f. of self 
induction. The applied e.m.f. in this latter case is represented by 
the curve E a , Fig. 230, and it is important to note that, in the 
case of an inductive circuit of negligible resistance, the current / 
lags the applied voltage E a by 90 degrees. 

241. Voltage and Current Relations. — The current / in Fig. 230 
changes from — I m to + I m in the time of half a cycle or in 

^t. seconds so that 

E av , the average voltage of self induction between b and c, 

= L (average value of -r. J 

/2Ln\ 

= L 1 ) =4fLI m 

and E m ax, the maximum voltage of self induction 
= \y.E av = \ X4:fLI m 

= 2 irfLIm 
therefore E eff = 2 irfLI e ff 

In direct current circuits, E = IR. In inductive circuits of 
negligible resistance, E = IX where X, called the inductive 
reactance, is expressed in ohms and is numerically equal to 
2tt/L. 

An alternating e.m.f. of 110 volts sends 2.2 amperes through an inductance 
coil of negligible resistance at 60 cycles. Find the reactance at 60 cycles 
and find also the coefficient of self induction. 
X, the reactance = E/I 

= 110/2.2 = 50 ohms 

L, the coefficient of self induction = ~— e 

Zirj 

= 2^60 =0-133 henry 

If the voltage applied to the above circuit is kept constant at 110, find 
the current that will flow through the inductance at 30, 60, 90 and 120 cycles 

X, the reactance = 2tt/L = 50 ohms at 60 cycles, from last problem 
= 25 ohms at 30 cycles 
= 75 ohms at 90 cycles 
= 100 ohms at 120 cycles 
J, the current = E/X = 110/25 = 4.4 amp. at 30 cycles 
= 110/50 = 2.2 amp. at 60 cycles 
= 110/75 = 1.47 amp. at 90 cycles 
= 110/100 = 1.1 amp. at 120 cycles 



Art. 243] 



INDUCTIVE CIRCUITS 



209 



The current is inversely proportional to the frequency because, the greater 
the frequency the smaller the current required to give the same voltage of 
self induction. 

242. Power in an Inductive Circuit. — The power in a circuit 
at any instant in watts is the product of e and i the voltage and 
current at that instant. In an inductive circuit of negligible 
resistance the current lags the applied voltage by 90 degrees and 
the curves representing e and i are shown by light lines in 
Fig. 231. 

At the instants a and b the voltage is zero so that the power 
is zero at these instants: it is also zero at instants g, d and / 




Fig. 231. — Voltage, current and power in an inductive circuit. 

when the current is zero. Between g and a the voltage and cur- 
rent are in the same direction so that power is positive or energy 
is being put into the circuit, while between a and d the current 
and voltage are in opposite directions so that power is negative 
or energy is being taken from the circuit; the average power 
in the circuit is zero. 

A hypothetical mechanical circuit with somewhat similar 
properties is shown in Fig. 232. As the weight W falls, work is 
done on the flywheel and the velocity increases until the rope is 
all unwound; the flywheel continues to rotate and now raises 
the weight, so that work is done by the flywheel until the weight 
has been lifted to the original position, the same cycle is then 
repeated. During one half cycle the power is positive or energy 
is put into the flywheel while during the next half cycle the power 
is negative or energy is being taken from the flywheel, so that the 
average power is zero. 

243. Examples of Inductive and Non-inductive Circuits. — 

The coefficient of self induction L = T 10 ~ 8 so that, to have a 

14 



210 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxix 



large inductance, a circuit must be linked by a large flux <j> for 
a given current /. The inductance of coil B Fig. 233 is much 
greater than that of the duplicate coil A because the flux <£ 
has been greatly increased by the addition of the iron core C. 

One form of adjustable inductance is shown in Fig. 234. 
If the iron cross piece mn is brought nearer to the poles pq, the 




I 


\+\ 


f 

f 









Wood t 


V 


C 








> 
> 


1 

i 

i 

j 


> 




> f 




» c 




> c 












» 














v - 









Fig. 232. 



B 
Fig. 233. — Inductive circuits. 



reluctance of the magnetic circuit is decreased, so that a larger 
flux <f> is produced by a given current I and the inductance is 
thereby increased. 

An incandescent lamp filament has an inductance which is 
negligible compared with its resistance. The number of turns 







I V 








> 


' 






> 
























c 


























V 




V 













Fig. 234. — Adjustable inductance. 



linked is small, while the flux has to pass through a path con- 
taining no iron and moreover is produced by an exciting coil 
having only two or three turns, so that 4> is small and L = 
(ncf>/I)10-* is negligible. The resistance on the other hand is 
high, that of a 16 candle-power carbon lamp being about 200 ohms. 



Art. 244] 



INDUCTIVE CIRCUITS 



211 



While a transmission line has only one turn, its inductance 
is not negligible because that one turn is very long and is linked 
by a large flux <f> particularly if the wires are spaced far apart 
because then, as shown in Fig. 235, there is room for a large 
flux to pass between the wires. 




Diagram C 



Diagram B 



Wires far apart. Wires close together. 

Fig. 235. — Flux linking a transmission line. 

A simple long loop such as that in Fig. 236 has a negligible 
inductance because there is no room for flux to pass between the 
wires. Non-inductive resistances are made in the form of a 
long narrow loop and are then coiled up for convenience as shown 



a , 



A B 

Fig. 236. — Non-inductive resistance. 



1 1§ 




jjiiiil \ 1 




^ 




J| f ■ ! ' f 


y 


l "1 


x 



in diagram B. The resistance between a and b may be large, 
but the inductance is negligible. 

244. Voltage, Current and Power in Resistance Circuits. — If 
an alternating voltage is applied to a non-inductive circuit of 
resistance R then, since there is no e.m.f of self induction oppos- 
ing the change of current, the current i at any instant = e/R 



212 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxix 

and increases and decreases with the voltage, oris in phase with 
the voltage, as shown in Fig. 237. 

The power in such a circuit is zero at the instants a, b, c and d 
when both voltage and current are zero but is positive at all other 
instants, that is energy is put into the circuit but none is taken out 
again. The average power 

= the average value of ei 

= the average value of E m sin X I m sin 6 

= Eml m (average value of sin 2 6) 

m m -i no 

= 2 , seepage 198 

J-j m J- m 

= 1~2 X V2 

= EI and is also = PR since E = IR 




Fig. 237. — Voltage e, current i and power e X tin a resistance circuit. 



where E and / are effective values, see page 198. Thus, in a non- 
inductive circuit, the power is the product of the effective voltage 
and the effective current. 

245. Resistance and Inductance in Series. — If an alternating 
current I is flowing in a circuit with a resistance R and a reactance 
X in series, as shown in Fig. 238, then (alternating voltages E r = 
IR and E x = IX will be found across the two parts of the circuit. 
The applied voltage E is the vector sum of the two components 
E r and E x and may be determined as follows: 

A vector / is drawn in any direction. 

A vector E r = IR is drawn to scale and in phase with I, see 
above. 

A vector E x = IX is drawn to scale in such a direction that I 
lags E x by 90 degrees, see page 207. 



Art. 24(5] 



INDUCTIVE CIRCUITS 



213 



Then E = the vector sum of E r and E x 

= < e 2 +1& 

= ^T( IR) 2 + ( IX) 2 

= I ^Ir 2 + X 2 

The current now lags the applied voltage by an angle a, as 
shown by the vectors and by the curves in Fig. 238. The power 
curve eXiis also shown from which it may be seen that, although 
the power is negative during a portion of the cycle yet the average 
power is positive. 




—E- x - 



-E^ 



-r-^tfuW^-A/W— i 



X 




9-1 



Fig. 238. 



-Voltage, current and power in a circuit which has resistance and 
inductance in series. 



The voltage E is the resultant of two components one of which 
E r = E cos a is in phase with I while the other component E x = 
E sin a leads I by 90 degrees. The average power due to the in 
phase component = (E cos a) X /, see page 212; that due to the 
other component is zero, see page 209, so that the total average 
power = EI cos a. 

246. The power factor in an alternating-current circuit is 

, n , ; , , . actual power 

denned as the ratio £-- ■ 

apparent power 

In any circuit in which the phase angle between the voltage E 

and the current 7 is a degrees then 



214 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxix 



the apparent power 
but the actual power 



= EI watts 
== EI cos a watts 
EI COS a 



therefore the power factor = 

= cos a 
and can never be greater than unity. 

If a resistance of 25 ohms and an inductive reactance of 50 ohms at 60 
cycles are put in series across 110 volts; find the current, the voltages across 
the two parts of the circuit and the power in the circuit at 30, 60 and 120 
cycles. 

The work is carried out in tabular form as follows : 



Frequency 


R ohms 


X = 2w/L 


VR 9 -+X* I E r i E x 


cos a 


Watts 


30 


25 


25 


35.4 3.10 78 i 78 


0.71 


240 


60 


25 


50 


56.0 1.96 49 98 


0.44 


96 


120 


25 


100 


103.0 1.07 27 107 


0.24 


29 



247. The Wattmeter. — Power in alternating- current circuits 
may be measured by means of an electrodynamometer type of 



Fig. 239. — Wattmeter connections. 



instrument called a wattmeter, constructed as described on page 
199, and connected as shown in Fig. 239. The line current I is 
passed through the stationary coils A, while the current which 
passes through the moving coil C is proportional to the voltage E 
and is in phase with it since the inductance of the coil C is negligi- 
ble compared with the additional resistance r. 

Since the moving coil C is carrying current and is in a magnetic 
field it is acted on by a force tending to turn it about a vertical 
axis and this force is proportional to the current in the coil and to 
the strength of the magnetic field. When the instrument is 
connected as shown in Fig. 239, the magnetic field is proportional 
to the current / while the current in the moving coil is propor- 
tional to the voltage E, and the average turning force is propor- 



Art. 248] INDUCTIVE CIRCUITS 215 

tional to the average value of e X i or to EI cos a, the average 
power in the circuit. 

If in the circuit shown in Fig. 239 

E = 100 volts 

/ = 50 amp. 

W = 4000 watts, measured bj' a wattmeter. 

. . actual power 

then the power factor of the circuit = r 

1 apparent power 

4000 

~ 100X50 

= 0.8 
and the phase angle between current and voltage is the angle whose cosine is 
0.8 or is 37 degrees. 

248. Transmission Line Regulation and Losses. — A transmis- 
sion line has resistance and inductance and mav therefore be 



■srsw* — > — w\Mr 




—£ uuuu ;» vvvvv A^ 

.[Eg X R \Et 

-* 'TRRnnp — <- — wwv *- 

a IR ' J b 

A B 

Fig. 240. — Vector diagram for a transmission line. 

represented as in Fig. 240. E g , the voltage at the generating 
station, is the vector sum of the terminal voltage E h the resistance 
drop IR and the reactance drop IX; the phase relation between 
these voltages is shown in diagram B. 

A vector I is drawn in any direction. 

A vector E t is drawn to scale equal to the receiver voltage, the 
angle a depending on the resistance and inductance of the load 
connected to the line; 

A vector E r = IR is drawn to scale in phase with I. 

A vector E x = IX is drawn to scale in such a direction that / 
lags E x by 90 degrees. 

The vector E g is the vector sum of E t , E r and E x and may be 
scaled off or determined by calculation. 

Since there is no power loss in the inductance of the line, the 
total loss is in the resistance and is equal to PR watts. 

Values of line resistance and line reactance are generally given 
in ohms per mile as in the following table: 



216 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxix 



Size of wire 


Resistance 

1 


Inductive 
reactance per mile of wire 


at 60 cycles 


B. and S. 
gauge 


Cir. mils 


Ohms per 
mile of wire 


24" 


48" 


72" 


96" spacing in 
inches 


0000 


211600 


0.258 


0.594 


0.678 


0.728 


0.763 


000 


167800 


0.326 


0.608 


0.692 


0.742 


0.776 


00 


133100 


0.411 


0.622 


0.706 


0.756 


0.790 





105500 


0.518 


0.637 


0.721 


0.770 


0.804 


1 


83690 


0.653 


0.650 


0.735 


0.784 


0.819 


2 


66370 


0.824 


0.665 


0.749 


0.797 


0.833 


4 


41740 


1.309 


0.693 


0.777 


0.826 


0.860 


6 


26250 


2.082 


0.721 


0.805 


0.854 


0.889 



75 kw. at 2200 volts and 60 cycles has to be delivered at the end of a 5- 
mile line, the size of wire being No. B. and S. gauge and the spacing 48 
in. Find the voltage in the generating station and the power loss in the 
line if the load current is lagging arid the power factor is 0.8. 

watts = E t X I X cos a, see page 214, 
or 75 X 1000 = 2200 X I X 0.8 
and the current = 42.5 amp. 
the resistance = 0.518 ohms per mile of wire 

= 5.18 ohms for a 5-mile line 
the reactance = 0.721 ohms per mile of wire at 60 cycles 
= 7.21 ohms for a 5-mile line 
IR = 42.5 X 5.18 = 220 volts 
IX = 42.5 X 7.21 = 307 volts 
ab, Fig. 240 = (2200 X 0.8) + 220 = 1980 
be, Fig. 240 = (2200 X 0-6) + 307 = 1627 . 

E a = V(ab) 2 +{bc) 2 = a/1980 2 + 1627 2 
= 2560 volts 

g& 1980 nnnr 
the power factor at the generating station = — = onan = 0./75 

the power put into the line = 2560 X 42.5 X 0.775 = 84.4 kw. 

the power delivered = 75 kw. 

the loss in the line = 84.4 — 75 = 9.4 kw. 

and this is equal to PR = (42.5) 2 X 5.18 = 9.4 kw. 

249. Resistance and Inductance in Parallel. — If an alternating 
voltage E is applied to a circuit which has a resistance R and a 
reactance X in parallel, as shown in Fig. 241, then a current 
I r = E/R will flow through the resistance and will be in phase 
with E while a current I x = E/X will flow through the reactance 
and will lag E by 90 degrees. 

These two currents are drawn to scale in diagram B and the 
total current I is the vector sum of I r and I x and 



v/ r 2 + /; 



Art. 249] 



INDUCTIVE CIRCUITS 



217 



If a resistance of 25 ohms and an inductive reactance of 50 ohms at 60 
cycles are put in parallel across 110 volts, find the current in each part of the 
circuit and also the total current at 30, 60 and 120 cycles. 




A B 

Fig. 241. — Vector diagram for a parallel circuit. 

The work is carried out in tabular form as follows : 



Frequency 



R ohms 



2wfL 



Ir 



cos a Watts 



30 

60 

120 



25 
25 
25 



25 


4.4 


4.46.200.710 


485 


50 


4.4 


2.24.930.890 


485 


00 


4.4 


1.14.560.965 


485 



CHAPTER XXX 
CAPACITY CIRCUITS 

250. Condensers. — Two conducting bodies separated by insu- 
lating material form what is known as an electrostatic condenser. 
In diagram A, Fig. 242, a and b, two plates of a condenser, are 
at the same potential. When the switch k is closed a momentary 
current i passes in the direction of the arrows in diagram B and 
the condenser is said to be charged; the potential of plate a is 
raised to that of the positive line terminal, the potential of plate 
b is lowered to that of the negative line terminal and the differ- 
ence of potential between the plates becomes equal to the line 
voltage. 





.A - Condenser B - Condenser Charging C- Condenser Discharging 

without Charge and Storing Electricity and giving up Electricity 

Fig. 242. — Charge and discharge of a condenser. 

If the switch k is now opened, the voltage between the plates 
remains unchanged and a quantity, or charge, of electricity re- 
mains stored in the condenser. 

To make the condenser give up its charge, the insulated plates 
must be connected by a conducting material, such as the wire d 
in diagram C, so as to bring them to the same potential. When 
this is done, a momentary current passes in the direction shown, 
from the positive to the negative plate. 

The quantity of electricity stored in a condenser, called the 
charge, is equal to the average current flowing into the condenser 

multiplied by the time during which it flows or is equal to I idt 

where i is the charging current at any instant. In any condenser 
this charge is found to be directly proportional to the applied 
voltage or 

q = Ce 
218 



Art. 251] 



CAPACITY CIRCUITS 



219 



where q is the charge in coulombs (amperes X seconds) 
e is the applied voltage 

C is a constant called the capacity of the condenser 
and is expressed in farads. 

A condenser of 1 farad capacity will hold a charge of 1 coulomb 
if a difference of potential of 1 volt is applied between the plates. 

The capacity of a condenser of given dimensions is found to 
depend on the insulating material, or dielectric, between the 
plates. If a condenser with air as dielectric has a capacity of 
F farads then the capacity becomes equal to kF farads when 
another dielectric is used. The constant k is called the specific 
inductive capacity of the material. Average values of k are 
given in the following table for the materials generally used 
in commercial condensers. 



Material 


Specific 


inductive capacity 


Air 




1 


Glass 




4 (varies considerably with the 
quality of the glass) 


Mica 




6 


Paraffined pa 


per 


2 



251. Capacity Circuits with Direct and with Alternating 
Currents. — A capacity circuit is one which contains a condenser. 




A B 

Fig. 243. — Flow of current in a capacity circuit. 

If a constant e.m.f . is applied across the terminals of the circuit 
shown in Fig. 243, then a momentary current will flow in the direc- 
tion shown in diagram A to charge the condenser but current 
will not flow continuously since the circuit is broken by the insu- 
lating material between the plates. 

If the applied voltage is now reversed, current will flow in the 
direction shown in diagram B until the condenser has given up 
its charge, and will continue to flow in this direction until the 
condenser is re-charged in the opposite direction. If then the 
applied voltage is alternating, a charging current will flow in and 



220 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxx 

out of the wires x and y with a frequency which is the same as 
that of the applied voltage, and the lamps L will light up if 
sufficient current flows to heat the filaments. 

The greater the capacity of a condenser, the more electricity 
it can hold, and the larger the charging current that passes 
through the connecting wires. Furthermore, the greater the 
frequency of the applied voltage, the shorter the time available 
in which to charge the condenser, and therefore the larger the 
current that must flow. With a given applied alternating vol- 
tage, and therefore with a definite alternating charge (since q = 
Ce), the alternating current i is proportional to the capacity, 
and to the frequency or 

i = a const. X C X / 

the constant will be determined later. 

252. Phase Relation between Voltage and Current in Capacity 
Circuits. — If the voltage applied to the condenser in Fig. 244 be 
represented by curve E then the charge, which is proportional to 
the voltage, is represented by the curve Q; the condenser is 




Emf. is -±J Charge is 

Decreasing - \b Decreasing' 

Fig. 244.: — Phase relation between the voltage and current in a capacity 

circuit. 

charged alternately in opposite directions, thus between the 
instants m and n the plate a is positive while between n and p 
the plate a is negative. 

At the instants q and r the charge in the condenser is not chang- 
ing, the currents in the leads x and y must therefore be zero. 

Between m and q the voltage and the charge are increasing and 
current flows in the positive direction, from the positive to the 
negative terminal as shown in diagram A, until, at the instant 
q, the charge is complete and the current has become zero; this 
gives the part fq of the current curve, see diagram C . 

Between q and n the voltage and the charge are decreasing so 
that current must now be flowing out of the condenser or in the 



Art. 253] CAPACITY CIRCUITS 221 

negative direction as shown in diagram B; this gives the part 
qg of the current curve. 

During the next half cycle between n and p the condenser 
charges and discharges in the opposite direction, so that the cur- 
rent curve gh is the same as the curve fg except that the sign is 
reversed. 

From these curves it may be seen that the current leads the vol- 
tage by 90 degrees. 

In the above discussion of phase relation between current and 
voltage in capacity circuits it is assumed that the current has been 
flowing for a few seconds. It is obvious that, at the instant the 
switch in a circuit is closed, the current in that circuit must be 
zero no matter what value the e.m.f. may have, so that the cur- 
rent waves are generally abnormal for a few cycles after the clos- 
ing of the switch, but they gradually change and become regular 
waves leading the e.m.f. by 90 degrees. 

253. Voltage and Current Relations in Capacity Circuits. — The 
charge in the condenser shown in Fig. 244 changes from zero to 
Q m = CE m coulombs in the time of one-quarter of a cycle, or 
in 1/4/ seconds, so that, since charge = average current X time, 

therefore Q m = CE m = I av X ^, 

and I av , the average charging current = 4fCE m amp. 

Now the maximum charging current I m = I av X <->> see page 197. 

= I X ±fCE m 
= 2TfCE m 

therefore /, the effective current in a capacity circuit 

= 2wfCE where E is the effective voltage. 

The physical meaning of this equation was explained on 
page 220. 

In direct-current circuits E = IR ; in capacity circuits E = IX 
where X, called the capacity reactance, is expressed in ohms and 

is numerically equal to 9 .„ 

An alternating e.m.f. of 110 volts sends 2.2 amp. through a capacity 
circuit at 60 cycles. Find the reactance at 60 cycles and find also the capac- 
ity of the condenser. 



222 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxx 

X, the reactance = E/I 

= 110/2.2 = 50 ohms. 

C, the capacity = , v 

z-kja. 

— o~ an m = 5.3 X 10 -5 farads = 0.53 microfarads. 

If the voltage applied to the above circuit is kept constant at 110, find the 
current that will flow through the capacity at 30, 60, 90 and 120 cycles. 

X, the reactance = 2 „„ = 50 ohms at 60 cycles, from last problem 

= 100 ohms at 30 cycles 
= 33.3 ohms at 90 cycles 
= 25 ohms at 120 cycles 

I, the current = E/X= 110/100 = 1.1 amp. at 30 cycles 

= 110/50 = 2.2 amp. at 60 cycles 

= 110/33.3 = 3.3 amp. at 90 cycles 

= 110/25 = 4.4 amp. at 120 cycles 

254. Parallel Plate Condenser. — As shown in the last problem, 
a condenser with a capacity of 0.53 microfarads will take a cur- 




Fig. 245. — One method of constructing condensers. 

rent of 2.2 amp. at 110 volts and 60 cycles. It is desirable to 
know the approximate dimensions of such a condenser. 

The capacity of a parallel plate condenser is given by the 
formula 

~ . - , 1 1 kA 

Cmfarads = S x 93aon x ~r 

where A is the area of the active surface of one plate in sq. cm. 
t is the distance between plates in cm. 
k is the specific inductive capacity, see page 219. 

A condenser constructed as in Fig. 245 has plates of tin foil which are 40 
ft. long and 3 in. wide and are separated by paraffined paper 0.0025 in. thick. 
Since both sides of each plate are active, 

A =2X40X12X3= 2,880 sq. in. 
= 18,600 sq. cm. 



Art. 256] CAPACITY CIRCUITS 223 

t = 0.0025 in. = 0.0063 cm. 

k = 2.0 

, , „• , , 1 1 18600 

therefore C in farads = -^ X Twiqu X q QQgg X 2 

= 0.53 X 10" 6 farads 
= 0.53 microfarads 

Such a condenser will go into a tin case 1.75 in. square by 4 in. deep. 

255. Power in Capacity Circuits. — The power in a circuit at 
any instant is the product of e and i the voltage and the current at 
that instant. In a capacity circuit the current leads the applied 
voltage by 90 degrees and the curves representing e, i and ei are 
shown in Fig. 246. This latter curve is obtained by multiplying 
together corresponding values of e and i at different instants; 




Fig. 246. — Voltage e, current i and power e X i in a capacity circuit. 

at m and n the voltage and therefore the power are zero; the 
power is also zero at instants q and r when the current is zero. 
Between m and q energy is stored in the condenser while between 
q and n the same energy is given up by the condenser, so that the 
average value of the energy used is zero and so also is the average 
power in the circuit. 

256. The Formulae used in Circuit Problems are : 
Resistance Circuit: 
E = IR 

current is in phase with voltage, see page 211 
power = EI watts. 
Circuit with Inductive Reactance : 
E = IX where X = 2wfL, see page 208 
current lags voltage by 90 degrees, see page 207 
power is zero. 



224 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxx 
Circuit with Capacity Reactance : 

E = IX where X = 7T7r/> see P a S e 221 
Ztt/O 

current leads voltage by 90 degrees, see page 220 

power is zero. 



R t l I 



,.E 
'I 



<>E 



■*-I 



kE 



esistauc.e 


Inductive 


Capacity 


Circuit 


Circuit 

Fig. 247. 


Circuit 



257. Resistance, Inductance and Capacity in Series. — In the 

solution of such a circuit as that shown in Fig. 248, the current 
vector has to be taken as a basis for phase relation since it is the 
same in all three parts of the circuit. The voltage E is the vec- 
tor sum of E r , Ei and E c and is determined as follows : 

A vector 7 is drawn in any direction. 

A vector E r = IR is drawn to scale in phase with 7. 

A vector Ei = IXi is drawn to scale in such a direction that 7 
lags Ei by 90 degrees. 

A vector E c = IX c is drawn to scale in such a direction that I 
leads E c by 90 degrees. 

Then E = the vector sum of E r , E L and E c 

= V# r 2 + (Ei- E c y 



= V (712)8+ (IX i- IX c y 

= i V#2 + (x,~ x c y 

and the current will lead or lag the applied voltage according as 
X c is greater or less than X h 

When X c = Xi the capacity and the inductive reactances 
exactly neutralize one another and the current has its maximum 
value and is equal to E/R. The circuit is then said to be in 
resonance. 



Art. 257] 



CAPACITY CIRCUITS 



225 



The inductive reactance X t is directly proportional to the fre- 
quency and is equal to 2irfL whereas the capacity reactance X c is 
inversely proportional to the frequency and is equal to l/2irfC. 
If then in Fig. 248, the voltage E across the terminals is kept con- 
stant and the frequency is increased, Xi will increase and X c will 
decrease until when 

Xi = X c 



or 



and 



2tt/L = 



1 

27T/C 

1 

2rVLC 



+-—E r - 



*M*SH 



__AAAMA-W5WJ^-| U- 



Xj 



El-Ec 



El 



E r 



■*-! 




Frequency 

Fig. 248. — Voltage and current in a circuit with resistance R, inductive 
reactance X e and capacity reactance X c in series. 



the circuit is said to be in resonance and the current has its 
maximum value. 

The same problem is found in mechanics. An alternating 
force applied to a spring will cause the spring to oscillate. As 
the frequency of the applied force increases, the amplitude in- 
creases and reaches its maximum value when the applied fre- 
quency is the same as the natural frequency of vibration of the 
spring; with further increase of the frequency, the amplitude 
will decrease. This principle is made use of in the instrument 
shown in Fig. 215. 

15 



226 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxx 

1 



The frequency / = 



is called the frequency of resonance 



2ttVLC 
and is also called the natural frequency of the circuit. 

If a resistance of 2 ohms, an inductive reactance of 12 ohms at 60 cycles 
and a capacity reactance of 20 ohms at 60 cycles are put in series across 110 
volts plot the current / and also the voltages E r , Ei and E c against fre- 
quency. 

Several points for these curves are determined as follows : 













1^5 




H 








>> 

o 

C 

a 1 


c$ 






*~5 

H 

I 

g 


1 

is 

1+ 
[Si 

> 


1 L 
+ 
> 


03 

II 

Mi 






20.0 


2 


4.0 


60.0 


-56 


56 + 


1,97 


3.94 


7.9 118 


40.0 


2 


8.0 


30.0 


-22 


22.1 


4.98 


9.96 


40.0 150 


60.0 


2 


12.0 


20.0 


-8 


8.24 


13.4 


26.8 


161.0 268 


80.0 


2 


16.0 


15.0 


1 


2.24 


49.2 


98.4 


788.0! 738 


100.0 


2 


20.0 


12.0 


8 


8.24 


13.4 


26.8 


268.0 161 


77.5 


2 


15.5 


15.5 





2.0 


55.0 


110.0 


852.0! 852 



the frequency of resonance can be determined readily by trial and is 77.5 
cycles, because then Xi = X c = 15.5 ohms. 

If the circuit is in resonance and the resistance is low then a 
large current will flow, and if in addition the reactances are large 
then the voltage drops across these reactances are large and may 
have several times the value of the applied voltage, this result is 
shown in the above problem; the inductance coil and the con- 
denser must be insulated to withstand 852 volts and not merely 
the applied 110 volts. 

In a circuit which contains only resistance, E = IR 
in a circuit which contains only inductance, E = IX L 
in a circuit which contains only capacity, E = IX c 
in a circuit which contains all three in series, E = IZ 
where Z, called the impedence of the circuit = Vi£ 2 + (Xi — X c ) 2 

258. Resistance, Inductance and Capacity in Parallel. — In 
the solution of such a circuit as that shown in Fig. 249, the voltage 
vector has to be taken as a basis for phase relation since it is the 
same for all three parts of the circuit. The current I is the vector 
sum of /,., 1 1 and I c and is determined as follows : 

A vector E is drawn in any direction. 



Art. 258] 



CAPACITY CIRCUITS 



227 



A vector I r = E/R is drawn to scale in phase with E. 

A vector I t = E/Xi is drawn to scale and lagging E by 90 
degrees. 

A vector I c = E/X c is drawn to scale and leading E by 90 
degrees. 
Then / = the vector sum of I r , h and I c 

= V/ r * + (7, ZTJji 

= <{E/RY+{E/X l -E/X C Y 



-N(ir+(h-i) 




-c- J l 



II 




Frequency 

Fig. 249. — Circuit with resistance, inductance and capacity in parallel. 



and the current will lead or lag the applied voltage according as 
I c is greater or smaller than I t . 

When the circuit is in resonance, X c = Xi and the current in 
the line has its minimum value and is equal to E/R. If the re- 
actances are then low compared with the resistance R, the cur- 
rents 1 1 and I c may be much larger than the line current /as 
shown in the following example. 



228 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxx 

If a resistance of 20 ohms, an inductive reactance of 2.4 ohms at 60 cycles 
and a capacity reactance of 4 ohms at 60 cycles are put in parallel across 110 
volts, plot the currents in the different parts of the circuit against frequency. 

Several points for those curves are determined as follows : 



Frequency 


R 


Xl = 2irfL 


XC ~ 27T/C 


Ir ■ Il Ic 


II- Ic 


I 


20.0 


20 


0.8 


12.0 


5.5 1 137 


9 


128 


! 128 + 


40.0 


20 


1.6 


6.0 


5.5 j 69 


18 


51 


51.2 


60.0 


20 


2.4 


4.0 


5.5 i 46 


28 


18 


18.8 


77.5 


20 


3.1 


3.1 


5.5 35 


35 





5.5 


80.0 


20 


3.2 


3.0 


5.5! 34 


37 


- 3 


6.2 


100.0 


20 


4.0 


2.4 


5.5 1 28 


46 


-18 


18.8 



CHAPTER XXXI 
ALTERNATORS 

259. Alternator Construction. — The essential parts of a re- 
volving field type of alternator are shown in Fig. 250. The 




E. m. f. wave 

Fig. 250. — Revolving-field type of alternator. 




Si 




Fv 



Fig. 251. — Winding diagram. 




Fig. 252. — Alternator coil. 

stationary part which carries the conductors that are cut by the 
revolving field is called the stator; the revolving field system is 
called the rotor. 

229 



230 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxi 

The stator core B is built up of soft steel laminations and has 
slots on the inner periphery in which the stator coils are placed. 
One type of coil is shown in Fig. 252 and consists of several turns 
of copper wire which are insulated from one another and are then 
taped up with cotton and other such insulating material. The 
machine shown in Fig. 250 has four of these coils which are con- 
nected in series so that their voltages add up. 

Since a connection diagram such as Fig. 250 shows only one 
end of the machine, it is found desirable in practice to show the 
coils and connections by means of a developed diagram such as 
Fig. 251; this diagram shows what would be obtained if the 
winding in Fig. 250 were split at xy and then flattened out on a 
plane; the two diagrams are lettered similarly. 

The voltage between the terminals Si and F\ varies as shown 
in Fig. 250 and goes through four cycles per revolution. 

260. Two -phase Alternator. — In order to utilize more of the 
stator surface, a duplicate winding B is placed on the stator as 




Fig. 253. — Two-phase alternator. 

shown in Fig. 253. This machine has twice as many conductors 
as that in Fig. 250, but if the coils A and B are connected in 
series, it will be found that the voltage of the machine has not 
been doubled but has been increased only 41 per cent. 

It was pointed out on page 200 that the distance between two 
adjacent like poles is 360 electrical degrees, therefore the distance 
between similar points on windings A and B is 90 electrical 
degrees. If then the voltage generated in the four coils A in 
series is represented by the curve E a , Fig. 254, that generated 
in the four coils B in series has the same magnitude but lags E a by 
90 degrees and is therefore represented by the curve E b ; when the 
poles are in the position shown, for example, the voltage in the 



Art. 261] 



ALTERNATORS 



231 



winding A is a maximum while that in B is zero, these values are 
obtained at the instant c, Fig. 254. 

The resultant voltage when the two windings are connected in 
series is the vector sum of Ei and E 2 and is equal to V2# = 1.414i? 
and if 7 is the maximum safe current the conductors of the 
winding can carry then the maximum output is 1.414 EI watts. 
It is therefore desirable to use the two windings A and B as if 
they belonged to separate alternators and then, by dividing up 
the load between them as shown diagrammatically in Fig. 256, 
each winding can be made to deliver EI watts or the whole 
machine be made to deliver 2EI watts. 




Fig. 254. — Voltage curves of 
two-phase alternator. 



Fig. 255.— 
Voltage vector 
diagram for a 
two-phase al- 
ternator. 







i J 


E \ 






1 


— M 



Fig. 256. — Diagram- 
matic representation of 
a two-phase alternator. 



Since the voltage of winding B is out of phase with that of 
winding A, the machine operating as shown in Fig. 253 gives two 
phases of voltage and is called a two-phase machine, whereas that 
shown in Fig. 250 is a single-phase machine. The former machine 
requires four wires for the load while the latter requires only two. 

261. Three-phase Alternators. — If three similar and independ- 
ent single-phase stators are mounted beside one another as in 
Fig. 257 in such a way that their conductors are cut by the same 
revolving field, then three separate single-phase e.m.fs. may be 
obtained, one from each winding. If further these stators are 
mounted so that S h S 2 and >S 3 , the starts of the windings of the 
three phases, are spaced 120 electrical degrees apart, then the 
e.m.f . in winding B will reach a maximum 120 degrees after that 
in winding A has reached its maximum value, and the e.m.f. in 
winding C will lag that in winding B by 120 degrees, as,shown in 
Fig. 259. 



232 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxi 




Fig. 257. — Three single-phase stators. 




Fig. 258. — Three-phase alternator. 




Fig. 259. — Voltage curves of a three- 
phase alternator. 




Fig. 260. — Voltage'Vector 
diagram for a three-phase al- 
ternator. 



Art. 262] ALTERNATORS 233 

In practice the three windings are placed on the same core as in 
Fig. 258, but it must be noted that in the resulting three-phase 
machine the windings are independent of one another and supply 
distinct and independent e.m.fs. to three distinct and independent 
circuits so that the machine is exactly .equivalent to three separate 
single-phase machines and is therefore called a three-phase ma- 
chine. Part of the winding for such a machine is shown in Fig. 261 . 




Fig. 261. — Part of the stator of a large three-phase alternator. 

262. Y-Connection. — A three-phase machine is conveniently 
represented by a diagram such as that in Fig. 262, the three vec- 
tors in Fig. 260 being replaced by three separate and independent 
windings ; such a machine has six terminals and six leads, two for 
each phase. 

In order to reduce the number of leads, the three return wires a 2 , 
b 2 and c 2 may be connected together to form a single wire n. The 
current in this wire at any instant is therefore the sum of ii, i 2 and 
i 3 , the currents in the three phases. But it may be seen from 
diagram B that, at any instant, the sum of these three currents is 
zero; at instant a for example i\ is equal and opposite to i% + i% 
while at instant b, i x is equal and opposite to is and i 2 is zero, the 
wire n therefore carries no current and may be dispensed with. 
The resultant connection, shown in Fig. 263, is called the Y- 
connection and requires only three leads to supply the load, one 
lead always acting as the return for the other two. 

263. Delta-connection. — Another method of connecting the 
three windings of a three-phase machine is shown diagrammat- 
ically in Fig. 264, the windings being connected in series in the 



234 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxi 



|«-l-20->^l-2O^-l-2CM- 





S - Current Curves of a Three Phase 
Machine 



Fig. 262. — Diagrammatic representation of a three-phase machine. 





^VzE 



Ii=I 



Fig. 263.— Y-Connection. 





Fig. 264. — Delta-connection. 



Art. 264] 



ALTERNATORS 



following order SiF 2 ,S 2 F 3 , S^Fi. The wires making these connec- 
tions may be shortened and the terminals connected directly to 
one another as shown in diagram B, the slope of the vectors being 
unchanged. On account of the appearance of this latter diagram, 
this three-phase connection is called the delta-connection. 

Although the winding has been closed on itself, no current flows 
through this' closed circuit. The resultant voltage in the closed 
circuit is the sum of the voltages in the three phases, but it may be 
seen from diagram A, Fig. 262, that, at any instant, ei + e 2 + e 3 
is zero, the voltage in one phase being always equal and opposite 
to the sum of the voltages in the other two phases. If, however, 
an external circuit is connected between any two leads, then the 
voltage across that circuit will be E, the voltage of one phase, 
and current will flow through the circuit. 

264. Voltages, Currents and Power in a Y-Connected Machine. 
— If two coils SiFi and S 2 F 2 are connected as shown in Fig. 265 




Fig. 265. 



./L. -Et- 



K_ . -E i- . ->!<- - -E 2- - -x 

^mnp — I i?wvHi 

Si F\ ^ 

O 2 




--sJZE 



Fig. 266. — The vector difference between the two voltages E^and E 2 , each 
equal to E, is E t = y/5E. 

and the voltage E 2 lags Ei by 120 degrees then the resultant 

voltage E r is the vector sum of E 1 and E 2 and may be determined 

as shown in diagram A. 

If, however, the second coil is connected backward as shown in 

Fig. 266, then the resultant voltage E t is no longer the vector 

sum but is the vector difference and is obtained by reversing the 



236 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxi 

vector to be subtracted and then taking the sum as shown in 
diagram B, where 

E t = 2E cos 30 
= V3# 
= 1.7SE 

This latter connection is the Y connection, thus in Fig. 263, F\ 
and F 2 are connected together and the voltage between Si and S 2 
= 1.732?. The current I t in the line is the same as the current I 
in the winding. 

If the current I in each phase of the machine lags the voltage E 
of that phase by an angle a as shown in diagram B, Fig. 262, then 
the power in each phase = EI cos a watts 

the power in the three phases = 3EI cos a 

E,. 

1 1 COS 



_<V3, 
= ylSEJi cos a 
since E t = V32? and I t = I. 

265. Voltages, Currents and Power in a Delta-connected 
Machine. — If two coils SiFi and S 2 F 2 are connected as shown in 



Si Ft 



S 2 Fo 




Fig. 267. 



Si F x 

/i 



' fgF^ 




vi'j 



Fig. 268. — The vector difference between the two currents h and h, each 
equal to I, is Ii = s/sl. 

Fig. 267, and the current I 2 lags I x by 120 degrees then the result- 
ant current I t is the vector sum of h and I 2 and may be deter- 
mined as shown in diagram C. 

If, however, the second coil is connected backward as shown 



Art. 266] 



ALTERNATORS 



237 



in Fig. 268, then the resultant current Ii is no longer the vector 
sum but is the vector difference as shown in diagram D where 

h = 27 cos 30 
= V37 - 
= 1.73/ 
This latter connection is the delta-connection, thus in Fig. 264, 
Si and F 2 are connected together and the current Ii in the line 
connected to that point = 1.737. The voltage E t is the same as 
the voltage E of the winding. 

If the current 7 in each phase of the machine lags the voltage 
E of that phase by an angle a then 
the power in each phase = EI cos a watts 

the power in the three phases = 3EI cos a 

= 3E t ( — t= ) cos a 



V3 V 

= V3 EJi COS a. 

since E t = E and h = V32 
With the same current in the line and the same voltage between 
lines, the power is the same no matter whether the machine is 
connected Y or delta. 

266. Connection of a Three-phase Load. — The load on a three- 
phase line may be connected Y as in diagram A, Fig. 269, or 




A - Y- Connected B - Delta Connected C- Delta Connected 
Load Load Load 

Fig. 269. — Connection of the load to a three-phase circuit, 
delta as in diagram B. If the lamps shown are 100- watt lamps 
then, when Y-connected, the voltage per lamp is 110/V3 or 63.5 

1 .57 amp. When delta 



100 



volts and the current per lamp is r ^-- 



100 



connected, 110- volt lamps are required and they take =-=-n = ^-91 



238 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxi 

amp. The delta-connection is generally used for the connection 
of individual loads across the phases, the distribution circuits 
being connected to the power mains as shown in diagram C. 

If one of the lamps in diagram B burns out, the two remaining 
lamps will burn with their normal brilliancy, but if one of the 
lamps in diagram A burns out, then the two remaining lamps will 
be in series across 110 volts and will burn dimly since they are 
then operating at 55 volts instead of at the normal 63.5 volts. 

267. Power Measurement in Polyphase Circuits. — A poly- 
phase circuit is one with more than one phase. To measure the 



rt* 



W; 






v_®^^ 




W2 

3 Phase Circuit 



2 Phase Circuit 

Fig. 270. Fig. 271. 

r Figs. 270 and 271. — Wattmeter connections in polyphase circuits. 

power in a two-phase circuit, each phase must be considered 
separately and two wattmeters used as shown in Fig. 270. If 
the load is balanced, that is divided equally between the two 
phases, then only one set of instruments is required. 

If in a balanced two-phase circuit 

E = 100 volts 
I = 50 amp. 
W = 4000 watts 
then the total power = 4000 X 2 = 8000 watts 
the apparent power = (100 X 50) X 2 = 10,000 volt amperes 

the power factor = 8000/10,000 = 0.8 
and the current in each phase lags the voltage of that phase by an angle whose 
cosine is 0.8 or by 37 degrees. 

In a three-phase circuit it is usually impossible to reach the 
two leads of each phase and the power has to be measured out on 
the line where only three leads are available. One line, for ex- 
ample b, Fig. 271, is supposed to be the return line for the other 
two and two wattmeters are connected as shown to measure the 
power going out on these lines, the total power in the three-phase 
circuit is the sum of the readings obtained from the two meters. 



Art. 268] 



ALTERNATORS 



239 



If in Fig. 271 E = 100 volts 
/ = 50 amp. 
Wx = 5000 watts 
W 2 = 2500 watts 
then the total power = 7500 watts 
the apparent power = 1.73 X 100 X 50 = 8650 volt amperes. 

the power factor = 7500/8650 = 86.6 per cent, 
and the current in each phase lags the voltage of that phase by an angle whose 
cosine is 0.866 or by 30 degrees. 

268. Alternator Construction. — The construction of a revolving 
field type of alternator is shown in Fig. 272. 




Fig. 272. — Revolving-field type of alternator. 



The stator core B is built up of sheet steel laminations which 
are dovetailed into a cast-iron yoke A and clamped between two 
iron end heads E. These laminations have slots C on their 
inner periphery and in these slots are placed the armature con- 
ductors D which are insulated from the slots and are connected 
together to form a winding from which e.m.f. is supplied to an 
external circuit. The stator core is divided into-blocks by means 
of vent segments F and the ducts thereby provided allow air to 
circulate freely through the machine and keep it cool. 



240 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxi 

The rotor or revolving field system consists of a series of N 
and S poles carrying exciting coils H and mounted on an iron 
field ring. An alternator has to be excited with direct current, 
it cannot therefore be self exciting. The exciting current, gener- 
ally supplied by a small direct-current generator called an exciter, 
is led into the field coils through brushes M which bear on slip 
rings insulated from the shaft. 

The exciter voltage is independent of that of the alternator and 
is generally chosen as 120 volts so that, in the case of high- voltage 
alternators, the exciting current may be larger than the full-load 
current of the machine as in the following case : 

A single-phase alternator has an output of 1000 kw. at 13,200 volts and 
100 per cent, power factor, find the current at full-load. If the exciter vol- 
tage is 120 and the excitation loss is 2 per cent, find the output of the exciter 
and also the exciting current. 

a. Watts = volts X amperes X power factor, 
therefore 1000 X 1000 = 13,200 X amperes X 1.0 
and amperes at full-load = 76 

b. The exciter output = 2 per cent, of 1000 kw. = 20 kw. 

20 X 1000 
The exciting current = — "ion =167 amp. 

269. The revolving armature type of alternator is generally 
cheaper than the revolving field type of machine for small outputs 
at low voltages. Such a machine is shown diagrammatically in 
Fig. 273; the armature is the same as that of a direct- current 
generator except that the commutator is removed and the arma- 
ture is tapped at two diametrically opposite points m and n which 
are connected to slip rings 1 and 2. 

The e.m.f. between these slip rings is a maximum when the 
armature is in the position shown, and is zero when the armature 
has moved through quarter of a revolution from this position 
because then the voltages generated in the conductors between m 
and c are opposed by the equal voltages in the conductors between c 
and n. The e.m.f. again becomes a maximum after the armature 
has moved through half of a revolution from the position shown in 
Fig. 273 but the polarity of the slip rings is now reversed. The 
e.m.f. between the slip rings is therefore alternating and goes 
through one cycle per pair of poles passed. 

If the armature is tapped at four points as shown in Fig. 274, 
the voltage E x between the slip rings 1 and 2 is a maximum when 
the armature is in the position shown, while the voltage E 2 
between the rings 3 and 4 is zero at the same instant, and E 2 



Art. 270] 



ALTERNATORS 



241 



lags Ei by 90 degrees so that the machine is now a two-phase 
alternator. 

To obtain three-phase currents the armature must be tapped 
at three points as shown in Fig. 275, it then becomes a three 
phase delta-connected armature. At the instant shown, the 







Fig. 273.— Single-phase. Fig. 274.— Two-phase. Fig. 275.— Three- 
Figs. 273-275. — Revolving armature type of alternator. 




Fig. 276. — Inductor alternator. 

voltage E 2 is zero while E 1 is positive and decreasing and E 3 is 
negative and increasing, this corresponds to instant a on the 
voltage curve diagram. 

270. The inductor alternator, one type of which is shown dia- 
grammatically in Fig. 276, has been found suitable for the gener- 
ic 



242 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxi 



ation of high frequency e.m.fs., because of the simplicity of the 
mechanical construction. 

The stationary field coil F, when excited, produces a magnetic 
flux <f> which causes all the inductors N to have the same polarity. 
The coils C are cut by the lines of force as the inductors rotate, 
and the generated voltage is a maximum when the inductors are 
in the position shown and one side of each coil is cutting lines of 
force, the voltage is zero when the poles are in position y and is a 
maximum again but in the opposite direction when the poles 
are in the position z and the other side of each coil is now cutting 
the lines of force. The voltage therefore passes through one T 
half cycle while the inductors move from x to z or 10 cycles are 
passed through per revolution, so that a machine with five in- 




Fig. 277. — Magneto alternator. 

ductors is equivalent to a ten-pole revolving field machine. Since 
only one side of each coil is active at any instant in the case of the 
inductor alternator, it is the heavier of the two machines for a 
given output and is therefore used only when simplicity of con- 
struction is essential. 

271. Magneto Alternators. — Two types of alternating- current 
magnetos used for gas-engine ignition are shown in Figs. 277 and 
278. In the former machine, the armature coil C is stationary 
and the flux threading this coil is varied by the rotating inductor 
ab, whereas in the latter machine the coil is wound on the inductor 
and rotates with it. 

In each case, when the inductor is in the position shown, the 
flux passes through the coil C from a to b; half a revolution later 



Art. 271] 



ALTERNATORS 



243 



b is under the N pole and a under the S pole and the flux <f> now 
passes from b to a and therefore passes through coil C in the oppo- 
site direction. 

A high peak of e.m.f . is obtained from such machines by shaping 
the pole faces of the revolving parts so that the flux threading the 
coil C changes as shown in curve a, Fig. 279; the flux changing 
very rapidly as the inductor moves from under the poles into the 
neutral position. The e.m.f. in the coil C, being proportional to 
the rate of change of the flux, has then its maximum value as shown 




Fig. 278. 



—Magneto alter- 
nator. 



Fig. 279. — Voltage and current 
curves in a magneto alternator. 



in curve b, Fig. 279. The current lags the voltage by an angle a 
which increases as the reactance of the circuit increases and there- 
fore increases with the frequency of the alternator or the speed of 
the engine. 



CHAPTER XXXII 

ALTERNATOR CHARACTERISTICS 

272. Armature Reaction. — Part of the winding of an alter- 
nator is shown in Fig. 280, the poles being stationary and the 
field coils not excited. If an alternating current / is passed 
through this winding from an external source then lines of force 
will encircle the coils as shown. This magnetic field is alter- 
nating and induces in the coils an e.m.f . of self induction which 
opposes the applied e.m.f. and is equal to IX, see page 208, where 




Fig. 



280.— Magnetic flux due 
the armature current. 



to Fig. 281.— Dia- 
grammatic repre- 
sentation of an al- 
ternator. 



IE 

Fig. 282.— Vec- 
tor diagram for an 
alternator. 



/ is the current flowing and X is the reactance of the winding due 
to its self induction. An alternator may therefore be considered 
as a circuit with a resistance R and a reactance X as shown 
diagrammatically in Fig. 281. The value of X is generally from 
4 to 10 times the value of R. 

If now an alternator is operating under normal conditions, fully 
excited, generating voltage and supplying current then, of the 
total voltage generated, a portion IX is required to overcome the 

244 



Art. 273] 



ALTERNATOR CHARACTERISTICS 



245 



reactance of the winding and another portion IR to overcome the 
resistance; the terminal voltage E t is obtained from the generated 
voltage E by subtracting IR and IX as vectors. 

273. Vector Diagram at Full-load.— If in Fig. 282, E is the 
voltage generated by an alternator at no-load, and a circuit is then 
connected across the alternator terminals which takes a current I 
from the machine, the armature resistance drop IR is in phase 
with the current while the current lags the armature reactance 
drop IX by 90 degrees, see page 207, and the terminal voltage 
E t is obtained by subtracting IX and IR from E as shown in 
Fig. 282. 

Three cases are shown in Figs. 283, 284, and 285, in which an 
alternator has the same terminal voltage E t and delivers the same 





"Et 




Fig. 283.— Lagging Fig. 284.— 100 per Fig. 285.— Leading 

current. cent, power factor. current. 

Figs. 283-285. — Effect of the power factor of the load on the regulation of 

an alternator. 



current 7, but different circuits are used in the three cases so that 
the phase angles a are different. 

It may be seen from these diagrams that the regulation of an 
alternator, namely, the ratio (E — E t )/E t , depends largely on 
the power factor of the load, and becomes negative if the current 
is leading considerably, as shown in Fig. 285 where E t is greater 
than E . 

274. Regulation Curves of an Alternator. — Since the regulation 
of an alternator depends on the power factor of the load as well 
as on the current, the external characteristics have to be given 
with different power factors as shown in Fig. 286. These curves 
are generally determined by calculation after the resistance and 
reactance of the winding have been measured. 



246 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxii 



A single-phase alternator with an output of 416 amp. at 2400 volts has a 
resistance of 0.2 ohms and a reactance of 2.3 ohms. Find the regulation at 
100 per cent, power factor, and also at 80 per cent, power factor with a lag- 
ging current, the full -load voltage being 2400 volts in each case. 

Full-load current = 416 amp. 
the resistance drop IR = 416 X 0.2 = 83 volts 
the reactance drop IX = 416 X 2.3 = 960 volts. 
At 100 per cent, power factor, see Fig. 284 
Eo 2 = (Et + IR) 2 + (IX) 2 
= 2483 2 + 960 2 
and Eo = 2660 

E -Et 260 

10.8 per cent. 



the regulation 



E t 



2400 



30 % Power Factor, Leading 






3 





Armature Current 

Fig. 286. — Regulation curves of an alternator. 
At 80 per cent, power factor with lagging current, see Fig. 283 

Eo 2 = ab 2 + be 2 

= (Et cos a + IR) 2 + (Et*sm a + IX) 2 
= (2400 X 0.8 + 83) 2 + (2400 X 0.6 + 960) 2 
and Eo = 3120 
, . 3120-2400 nn 
the regulation = ^loo = 3® P er cent. 

275. Experimental Determination of Alternator Reactance. — 

The no-load saturation curve in Fig. 287 is determined in the 
same way as for a direct-current generator, see page 70. The 
alternator is then short-circuited through an ammeter as shown 
in diagram B, and run at normal speed, while simultaneous read- 
ings are taken of the armature current I a and the exciting cur- 
rent If from which the short-circuit curve is plotted. 



Art. 275] 



ALTERNA TOR CHARACTERISTICS 



247 



From these two curves the reactance of the alternator may 
readily be determined. With an exciting current oa for example, 
the voltage generated by the alternator is ab = 2400 volts at no- 
load. With the same excitation and with the armature short- 
circuited, the terminal voltage is zero and the generated voltage 
ab is used up in sending a current ac = 1040 amp. through the 
resistance and reactance of the winding or 



and 



voltage ab = current ac X ^R 2 + X 2 

2400 = 1040 X V# 2 + X 2 
V# 2 + X 2 = 2.3 ohms 

from which X may be determined if the value of R is known, and 
this may readily be measured by passing a direct current I 
through the alternator winding and measuring the voltage drop 
E, since R = E/I. 




ZB 



QEo 



A. - Connection for No Load 
Saturation Test 



Qla 



B- Counection for Short Circuit Test 
Erciting Current If 

Fig. 287. — Determination of the impedence of an alternator. 

It may be seen from Fig. 287 that the reactance X decreases 
as the magnetic circuit of the machine becomes saturated. The 
reactance is caused by the flux produced by the armature current, 
as shown in Fig. 280. When the field coils are excited so that 
the main flux of the machine is passing through the poles, then 
a smaller additional armature flux is produced with a given 
armature current than when the poles are not excited, and a 
smaller armature flux produces a smaller reactance. 

1. A three-phase Y- connected alternator has an output of 240 amp. at 
2400 volts. With a certain field excitation the no-load voltage between 
terminals was 2400 volts and the current in each line on short-circuit was 
600 amp. The resistance of each phase is 0.2 ohms. Find the reactance 
per phase. 



248 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxii 

Et, the terminal voltage at no-load = 2400 volts 

E, the voltage per phase at no-load = 2400/V3, page 236 

= 1390 volts 
Ii, the line current on short-circuit = 600 amp. 

1, the current per phase on short-circuit = 600 amp., page 236 

Z, the impedence per phase = =2.3 ohms 

y v ■ y 600 

X, the reactance per phase = \/ Z 2 — R 2 

= \/2.3 2 - 0.2 2 

= 2.3- 

Find the regulation of this machine at full-load and 100 per cent, power 
factor, the full-load voltage between terminals being 2400 volts. 

FulHoad current in line = 240 amp. 

the full-load current per phase == 240 amp. 

the resistance drop IR per phase = 240 X 0.2 = 48 volts 

the reactance drop IX per phase = 240 X 2.3 = 550 volts 

the fulHoad voltage between terminals = 2400 volts 

the full-load voltage per phase = 2400/\/3 = 1390 volts 

then at 100 per cent, power factor, see Fig. 284 

Eo 2 = (1390 + 48) 2 + 550 2 
and Eo = 1540 volts per phase 

and the no-load voltage between terminals = 1540 X a/3 

= 2660 volts 
, . 2660-2400 
the regulation = 9400 = per cen *- 

2. A three-phase delta-connected alternator has an output of 240 amp. at 
2400 volts. With a particular field excitation the no-load voltage between 
terminals was 2400 volts and the current in each line on short-circuit was 600 
amp. The resistance of each phase was 0.6 ohms. Find the reactance per 
phase. - 

Et, the terminal voltage at no-load = 2400 volts 
E, the voltage per phase at no-load = 2400 volts, page 237 
Ii, the line current on short-circuit = 600 amp. 
I, the current per phase on short-circuit = 600/\/3, page 237 

= 346 amp. 
2400 
Z, the impedence per phase = Wac. = 6.9 ohms 

X, the reactance per phase = -\/6.9 2 — 0.6 2 

= 6.9- 

Find the regulation of this machine at full-load and 100 per cent, power 
factor, the full-load voltage between terminals being 2400 volts. 

Full-load current in the line = 240 amp. 
the full-load current per phase = 240/V3, page 237 

= 139 amp. 



Art. 276] 



ALTERNATOR CHARACTERISTICS 



249 



the resistance drop IR per phase = 139 X 0.6 = 83 volts 
the reactance drop IX per phase = 139 X 6.9 = 960 volts 
the full-load voltage between terminals = 2400 volts 
the full-load voltage per phase = 2400 volts, page 237 
at 100 per cent, power factor, see Fig. 284, 

Eo 2 = (2400 + 83) 2 + 960 2 " 

E = 2660 volts 
, . 2660-2400 Hnn 

the regulation = 2400 — = P er cen *- 

276. Automatic Regulators. — To maintain the voltage of an 
alternator constant, the field excitation must be increased as the 
armature current increases and as the power factor decreases. 
This cannot be done by adding series field coils as in the case of 
the direct- current generator, see page 74, because the line cur- 
rent is alternating and not suitable for excitation purposes. 




Fig. 288. — Automatic voltage-regulator. 

Automatic regulators are used with alternators. The essen- 
tial parts of such a regulator are shown diagrammatically in Fig. 
288. To keep the voltage E t constant, some means must be 
provided to close the contact c so as to short-circuit the resistance 
r and thereby increase the exciter voltage and also the exciter 
current i e when the alternator voltage is too low, and to open 
this contact and insert the resistance r in the exciter field circuit 
when the alternator voltage is too high. 

The contact c is opened by the electromagnet M on which are 
two opposing windings A and B. When the contact a is open, B 
alone is excited and opens the contact c against the tension of the 
spring d, but when a is closed, the coil A also is excited and neu- 
tralizes the pull of B and the spring d closes the contact c. 

If then the alternator voltage rises, the pull of the solenoid S is 



250 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxii 

increased and its plunger is raised so as to open the contact a, 
the coil A is then deenergized and B opens the contact c and 
inserts the resistance r in the exciter field circuit. The alternator 
voltage now drops, the pull of the solenoid S decreases, and the 
weight of the plunger closes the contact a and thereby excites 
coil A which neutralizes the pull of B, the spring d then closes the 
contact c and short-circuits the resistance r and thereby increases 
the field excitation. 

When additional attachments are added to this regulator to 
make it more sensitive, the voltage fluctuations on which the 
operation of the regulator depends cannot be detected on a sensi r 
tive voltmeter. 

277. Efficiency. — The losses in an alternator are the same as in 
a direct- current generator and consist of the stray loss, the arma- 
ture copper loss, and the field excitation loss. These losses are 
determined in the same way as for the direct- current machine, see 
page 97. It must be noted however that the efficiency depends 
on the power factor of the load, as may be seen from the follow- 
ing example. 

In a two-phase alternator which has an output of 83 amp. per phase at 
2400 volts 

the stray power loss = 16 kw. 

the exciting current at full-load = 68 amp. at 100 per cent, power factor 

= 85 amp. at 80 per cent, power factor 
the exciter voltage is 110 and no automatic regulator is supplied 
the resistance of each phase of the armature winding, measured by 
direct current = 0.4 ohms. 

Find the efficiency when the power factor of the load is 100 per cent, and 
also when 80 per cent.; find also the horse-power of the driving engine. 

at 100 per cent, power factor at 80 per cent, power factor 

The output = 2 X 2400 X 83 = 2 X 2400 X 83 X 0.8 

= 400 kw. = 320 kw. 

The stray loss = 16 kw. = 16 kw. 
The excitation loss 

= 110 X 68 = 7.5 kw. = 110 X 85 = 9.4 kw. 
The armature copper loss 

= (83 2 X 0.4) X 2 =5.5 kw. = 5.5 kw. 

The total loss = 29 kw. = 30.9 kw. 

The input = 429 kw. = 350.9 kw. 
The horse-power of the 

driving engine = 575 h.p. = 470 h.p. 

400 320 

The efficiency = — — = 93.4 per cent. = =91.4 per cent. 

J 429 1 350.9 



Art. 278] ALTERNATOR CHARACTERISTICS 251 

The lower the power factor, the smaller is the power output, and 
at the same time the greater the excitation loss because of the 
increase in the exciting current required to maintain the voltage. 

278. Rating of Alternators. — An alternator is designed so as to 
give normal voltage and normal current without overheating, but 
the output in kilowatts will depend entirely on the power factor 
of the connected load. It is usual to specify the output at 100 
per cent, power factor and then, to emphasize the fact that this 
output cannot be obtained from the machine at lower power fac- 
tors, the unit of output is taken as the kilovolt ampere (kv.a.) 
and not as the kilowatt, where (kv.a. X power factor) = kw. 

A single-phase alternator can give 100 amp. at 2400 volts. What is the 
output of the machine in kv.a. and also in kw. if the power factor of the load 
is 80 per cent. 

i 2400 X 10 ... 

kva - = 1000 ~~ =240 

kw. = 240 X 0.8 = 192 

A three-phase alternator can give 100 amp. from each terminal with a vol- 
tage between terminals of 2400 then 

«, * + - i 1.73 X 2400 X 100 , OQ „ , 1K 

the output in kv.a. = inoo see P a S e ^6, = 415 

at 80 per cent, power factor the output would be 415 X 0.8 = 332 kw. 



CHAPTER XXXIII 
SYNCHRONOUS MOTORS AND PARALLEL OPERATION 

279. Principle of Operation of Synchronous Motors. — An 

alternating-current generator may be made to operate as a motor. 
When an alternating e.m.f . is applied to the winding of the single- 
phase machine shown in Fig. 289, an alternating current flows 
through that winding. The conductors a, b, c, and d are then 
carrying current and are in a magnetic field so that a force acts on 
each conductor, while an equal and opposite force acts on the 
poles and tends to turn the rotor. The current however is 
alternating, so that the force on the rotor is alternating in direc- 
tion unless the polarity of the poles is changed at the instant the 
current reverses. 




Fig. 289. — The synchronous motor. 
This would be the case if the machine was already running at 

such a speed that, during the time of half a cycle or in -^ seconds, 

At 
the rotor moves through the distance between two adjacent poles 
or through 1/p of a revolution. This speed, called the synchron- 
ous speed, is therefore equal to 

- X 2/ rev. per sec. 

120/ 

or rev. per mm. 

V 

252 



Art. 280] 



SYNCHRONOUS MOTORS 



253 



and is the speed at which the machine would have to run as an 
alternator in order to generate an e.m.f. of / cycles per second, see 
the formula on page 195. The table on page 195 therefore applies 
to synchronous motors, as this type of machine is called, as well 
as to alternators. When a synchronous motor is running at 
synchronous speed it is said to be in step with the alternators 
driving it. 

A synchronous motor is not self starting, but will develop a 
torque continuously in one direction if running at synchronous 
speed. Such a machine is generally brought up to speed by a 
small self-starting motor which is direct connected to the shaft- 

280. The Back E.m.f. of a Synchronous Motor. — If the motor 
M, Fig. 290, is rotating at synchronous speed, then its stator wind- 





Alternator 



Diagrammatic Bepresentation 
>- 






.Synchronous Motors 



A No Load B 

Fig. 290. — Alternator driving synchronous motors. 



ing is being cut by lines of force and an e.m.f. is generated in the 
machine in the same way as if it were driven by an engine. This 
e.m.f. E m , called the back e.m.f. of the motor, opposes the applied 
e.m.f. Eg and is of the same frequency, and, in the case where the 
two machines are equally excited, then E m = E g . 

If the motor is running on no-load, the load current that it takes 
from the line is practically zero, being merely sufficient to over- 
come the friction of the machine, and in such a case E m is exactly 
equal and opposite to E g at every instant as shown by the vectors 
in Fig. 292, for, under these conditions, there is no resultant e.m.f. 
and no flow of current through the machines. The poles of the 



254 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxii 

two machines will then rotate together with the relative position 
shown in diagram A, Fig. 290. 

If now the motor is loaded, it will slow down for an instant 
and will swing back relative to machine G as shown in diagram B, 
Fig.. 290. The back e.m.f. E m , although still equal to E g is now 
no longer opposite in phase as at no-load, but lags its no-load 
value by an angle 6 as shown by the vectors in Fig. 292. There 
is now a resultant e.m.f. E r which sends a current through the 
machines, and the torque developed due to this current keeps 
the motor running at synchronous speed but always with such a 
lag behind the generator as to allow a current to flow large enough 
to develop a driving torque equal to the retarding torque of the 
load. The motor therefore automatically takes from the genera- 
tor a current corresponding to the mechanical load. 



AE, 




M 




Fig. 291. — Mechanical anal- 
ogy to a synchronous motor. 



' ' E m Em 

No load. Full load. 
Fig. 292. — Voltage vector dia- 
grams for a synchronous motor. 



281. Mechanical Analogy. — The transmission of power by 
means of an alternator and a synchronous motor is similar in 
many ways to the transmission of power by means of a flexible 
spring coupling such as that shown in Fig. 291. If the load on 
the side M is increased, the spring stretches and M drops back 
through a small angle relative to G, but both continue thereafter 
to rotate at normal speed. 

282. Vector Diagram for a Synchronous Motor. — In Fig. 293 
E g is the e.m.f. generated in the alternator winding. 

E m is the back e.m.f. generated in the motor winding. 
E r , the resultant e.m.f., sends an alternating current 1 through 
both machines. 



Art. 283] 



SYNCHRONOUS MOTORS 



255 



R m and X m are the resistance and reactance of the motor winding 
R g and X g are the resistance and reactance of the generator 
winding. 

Er 

then 7 ~ TJ(R m +R g ) 2 +(X m +X g y 

and since the resistances are generally small compared with the 

reactances, see page 244, therefore 

X m -\-Xg 

Since the circuit is almost entirely inductive, the resistance being 
negligible, the current / must lag the voltage E r by 90 degrees. 
The power developed by the generator = E g I cos a. 



Generator [ ]£ 






Em 

Fig. 293. Fig. 294. Fig. 295.— Load greater than 

Light load. Heavy load. the maximum output. 

Figs. 293-295. — Vector diagram for the synchronous motor at various loads. 

If the load on the motor is now increased, the motor swings 
back relative to the generator by a greater angle 6, as shown in 
Fig. 294. The voltage E r and the current I are now larger than 
before and so also is E g I cos a the power developed by the 
generator and put into the circuit. 

283. Maximum Output. — An extreme case is shown in Fig. 295 
where the load has caused the motor to swing back by a large 
angle 6, but the power put into the line, namely E g I cos a, is 



256 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxiii 

now less than in the case represented in Fig. 294, so that, while 
the angle 6 increases as the load is increased, the power input 
E g I cos a increases only up to a certain point called the break- 
down point or point of maximum output. When the load exceeds 
this value the motor slows down and stops. 

The maximum output is generally more than twice the normal 
output of the machine as fixed by the heating of the windings. 

284. Operation of a Synchronous Motor when Under- and 
Overexcited. — In diagram B, Fig. 296, the excitation of the 
motor M is such that E m = E g . Two other conditions of opera- 
tion are represented in diagrams A and C. In the former case 
the motor is overexcited and, since the speed cannot change 
but must always be synchronous speed, the voltage E m must 

iEg 




ABC 
Fig. 296. — Effect of excitation on the power factor of a synchronous motor. 



increase with the excitation and must now be greater than E g . 
In the latter case the motor is underexcited and E m must be less 
than Eg. The load on the motor however is unchanged so that 
E g I cos a is constant. 

It is important to note that, in the case of the overexcited 
motor, diagram A, the current / leads the generator voltage E g , 
or an overexcited synchronous motor draws a leading current 
from the line. Now a condenser always draws a leading current, 
see page 220, so that an overexcited synchronous motor acts to a 
certain extent like a condenser. 

285. Use of the Synchronous Motor for Power Factor Correc- 
tion. — If the load connected to an alternator has a low power fac- 
tor, it is often advisable to arrange that some of the load shall be 



Art. 285] 



SYNCHRONOUS MOTORS 



257 



carried by synchronous motors so as to improve the power factor 
of the whole system. 

If 1000 horse-power of 2200-volt single-phase induction motors are 
operating at the end of a transmission line, find the current in the line and 
also the generator capacity required if the average power factor is 80 per 
cent, and the average efficiency is 90 per cent. (The induction motor, see 
Chap. 36, takes a lagging current, and its power factor cannot be controlled.) 

The output of the motors is 1000 h.p. 
1000 



the input to the motors is 



0.9 



= 1115 h.p. 

= 830 kw. 
830 



the generator capacity required = -~-~ = 1040 kv.a. 



the current in the line 



1040 X 1000 
2200 



= 473 amp. 



Et = 2200 




B 



Fig. 297. 



If 500 horse-power of the load is driven by a synchronous motor, the power 
factor of the whole system may be raised if this motor is overexcited and made 
to act as a condenser. 

If the power factor of the synchronous motor be made 80 per cent., with 
the current leading, then the vector diagram for the load is as shown in 
diagram B, Fig. 297. 



The induction motor output = 500 h.p. 

500 746 



the induction motor input 
the current for these motors 



0.9 X 1000 
415 X 1000 
2200 X 0.8 



415 kw. 
236 amp. 



The current for the synchronous motor also is 236 amp., but it leads by an 
angle whose cosine is 0.8 whereas the current for the induction motor lags by 
the same angle 

the resultant current in the line = 2 (236 X 0.8) 

= 378 amp. 

•, 378 X 2200 
the generator capacity required = innn 

= 830 kv.a. 
17 



258 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxiii 

By the use of an overexcited synchronous motor, the power fac- 
tor of the system is improved, the generator capacity required 
for the load is reduced, and so also is the current in the line, so that 
overexcited synchronous motors may be used with advantage at 
the end of a long line. 

286. Synchronizing. — Before a synchronous motor can carry 
a load it must be started and brought up to synchronous speed, 
and the switch S, Fig. 298, should not be closed until E m is equal 
and opposite to E g . If this switch were closed when E m and E g 
are acting in the same direction round the closed circuit, then the 
resultant voltage E r would be equal to E m + E g and would send 
a destructive current / through the circuit. When E m and j£ u 
are equal and opposite, then E r is zero, and no current / will 
flow when the switch S is closed. To test for this condition, a 
lamp L, or an indicating device called a synchroscope, is placed 




Fig. 298. — Alternator driving Fig. 299. — Alternators in par- 
a synchronous motor. allel. 



across the switch S. When the lamp is brightest, then E r = 
E m + E g ; when darkest, then E r = zero and the switch $ may 
be closed, after which the load may be put on the motor. The 
operation described above is called synchronizing. 

287. Hunting. — In the case of an engine-driven alternator, and 
particularly if the engine is a gas engine, the angular velocity is not 
uniform but consists of a uniform angular velocity with a super- 
imposed oscillation, the frequency of the generated e.m.f. there- 
fore is not constant, but rises and falls regularly. 

If this e.m.f. is applied to a synchronous motor, the synchron- 
ous speed of the motor tends to rise and fall regularly with the 
frequency, and the motor tends to have a superimposed oscilla- 
tion similar to that of the alternator. If the natural period of 
oscillation of the motor has the same frequency as this forced 
oscillation then the effect will be cumulative and the motor will 
oscillate considerably. 



Art. 288] SYNCHRONOUS MOTORS 259 

A similar result would be found with the model shown in Fig. 
291. If the torque applied to G is not uniform, then G will 
oscillate about its position of mean angular velocity and M will 
have an oscillating force impressed on it by the spring. If the 
moment of inertia of the flywheel M is such that its natural fre- 
quency of oscillation is the same as the frequency of the impressed 
oscillation then (rand M will swing backward and forward relative 
to one another through a considerable angle. 

As the two machines M and G oscillate relative to one another, 
the angle 0, Fig. 294, increases and decreases regularly, and the 
value of both E r and of the current / vary above and below the 
average value required for the load. This surging of current is of 
comparatively low frequency and is indicated by an ammeter 
placed in the circuit. Due to this surging, the circuit breakers 
protecting the machines may be opened although the load is not 
greater than normal, while the cumulative swinging of the ma- 
chines relative to one another, called hunting, may cause the 
motor to drop out of step. 

To prevent hunting, the impressed oscillations must be elim- 
inated or the natural frequency of vibration of the motor must 
be changed. The methods used in practice to minimize hunt- 
ing are: 

1. Dampen the governor if the impressed oscillations are found 
to be caused by a hunting governor. 

2. Change the natural period of vibration of the machine by 
changing the flywheel; the larger the moment of inertia of the 
rotating part of the motor, the longer is its natural period of 
vibration. 

3. Dampen the oscillations electrically by the use of pole 
dampers such as those described on page 295. 

288. Parallel Operation of Alternators. — Two alternators 
connected to operate in parallel are shown in Fig. 299. If the 
voltage of machine B is not exactly equal and opposite to that of 
A at every instant then current will flow in the local circuit 
between the two machines just as in Fig. 298. To prevent this 
the two machines must be synchronized in the same way as an 
alternator and a synchronous motor; when the lamp L in Fig. 299 
is dark, then E b is exactly equal and opposite to E a and the 
machines have the same frequency; the switch S may then be 
closed. If now the engine of B fails for an instant, then generator 
B will tend to slow down and will swing back relative to A, so that 



260 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxiii 

current will flow in the local circuit and B will be driven as a syn- 
chronous motor in the same direction as before and at the same 
speed. As soon as the engine of B recovers, the generator will 
swing forward again and carry its share of the load. 

If two direct-current generators are operating in parallel, and 
the field excitation of one of the machines is increased, then the 
voltage of that machine will be raised and it will take a larger 
portion of the load. An increase in load makes the engine and 
generator slow down and allows the engine to draw the additional 
amount of steam required for the additional load. 

If two alternators are operating in parallel, an increase in the 
excitation of one machine raises the voltage of that machine and 
tends to make it carry more of the load. But the machine can- 
not slow down and allow the engine to take more steam because it 
can run only at synchronous speed. An increase in excitation of 
one machine therefore increases its voltage and at the same time 
makes the current in that machine lag further behind the voltage 
so as to maintain the load EI cos a constant at the value corre- 
sponding to the steam supply. To change the distribution of 
load between two alternators operating in parallel, the governors 
of the driving engines must be manipulated so as to change the 
distribution of the steam supply. 

As the total load on the two alternators increases, they both 
slow down, and the engine governors automatically allow the 
necessary amount of steam to flow, while the frequency of the, 
generated e.m.f. decreases slightly. In order that the two 
machines may divide the load properly, the engines should have 
the same per cent, drop in speed between no-load and full-load. 

The same applies to alternators driven by water wheels. To 
make any one of a number of turbine-driven alternators take a 
larger portion of the total load, the governor of that machine 
must be manipulated to allow the turbine to take more water. 



CHAPTER XXXIV 



TRANSFORMER CHARACTERISTICS 



Secondary 



289. In order that electric energy may be transmitted 
economically over long distances, high voltages must be used; 
but in order that electric circuits may be safely handled, low vol- 
tages are necessary for distribution. The alternating- current 
transformer is a piece of apparatus by means of which electricity 
can be received at one voltage and delivered at another voltage 
either higher or lower. It consists essentially of two coils wound 
on an iron core; one coil receives energy and is called the primary 
coil, the other delivers energy and is called the secondary coil. 

290. Constant Potential Transformer.— In Fig. 300, C is a 
closed magnetic circuit on which 

are wound two coils having wi and 
n 2 turns respectively. When an 
alternating e.m.f. ei is applied to 
the coil w i while coil n 2 is closed 
through a circuit as shown, then a 
current ii flows in the primary coil 
and produces an alternating mag- 
netic flux </> which threads both 

coils and generates in them electromotive forces en and e 2 which 
are proportional to rii and n 2 the number of turns. 

Now e lb is called the back e.m.f. of the primary and, according 
to Lenz's law, opposes the change of the flux which produces it 
and therefore opposes ei which produces the change of flux; en 
is less than ei by the e.m.f. required to send the current i'i through 
the primary coil, which e.m.f. is small in modern transformers and 
seldom exceeds 1 percent, of e\ even at full-load. 

The e.m.f. e 2 sends a current i 2 through the secondary winding 
in such a direction as to oppose the change of the flux 4> which 
produces it, and therefore to oppose i\\ but the magnetizing 
effect of i\ must always be greater than the demagnetizing effect 
of ii by the amount necessary to produce the flux 4> in the mag- 
netic circuit. 

261 




Fig. 300. — The transformer. 



262 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxiv 

If now the impedence of the secondary circuit is decreased, the 
current i 2 will increase and thereby reduce the flux </>. But a 
reduction in <f> causes the back e.m.f. en, to decrease and thereby 
allow a larger current i\ to flow in the primary winding, so that 
the primary current always adjusts itself to suit the requirements 
of the secondary circuit. 

Now the primary resistance is so small that e ib and <t> do not 
drop more than about 1 per cent, between no-load and full-load, 
so the statement may be made that 4> is constant at all loads and 
therefore the resultant of n\i\ and n 2 i 2 , the primary and the 
secondary ampere-turns, must always be equal to some quantity 
wtfo which produces the constant flux </>. 

The quantity riiio may readily be found because, if the second- 
ary circuit be opened, no current can flow in that winding, and 
the current in the primary under these conditions has merely to 
produce the flux 0. This current must therefore be io and is 
called the magnetizing current of the transformer. 

Since e ib and e 2 are produced by the same magnetic flux <f> they 
are proportional to the number of turns n\ and n 2 , and since e-m is 
practically equal to ei therefore 

e 2 n 2 

and is called the ratio of transformation. 

It has also been shown that the resultant of ri\i\ and of n%%% 
must always be equal to ri\io where io, called the magnetizing 
current, is comparatively small, so that if io be neglected then 

riiii = n 2 i 2 

Wi i 2 ei 

or -- = - = — 

n 2 i\ e 2 

and eiii = e 2 i 2 

According to the law of conservation of energy, the input to a 
transformer must be exactly equal to the output, the losses being 
neglected (the efficiency of a 50-kilovolt ampere transformer is 
98 per cent.) therefore 

e x i\ cos a-i = e 2 i 2 cos a 2 
and e-iii = e 2 i 2 

therefoie cos a x = cos a 2 

so that, neglecting the magnetizing current and the losses, the 
power factor of the primary is the same as that of the secondary. 



Art. 292] TRANSFORMER CHARACTERISTICS 263 

291. Vector Diagram for a Transformer. — (a) No-load condi- 
tions. For a transformer which has a negligible no-load loss, the 
voltage and current phase relations are shown in Fig. 301. 

<f> is the magnetic flux threading both coils. 

to is the magnetizing current which produces </>. 

e 2 and ew, are the e.m.fs. generated in the coils n 2 and n-\ 
respectively, and lag the flux which produces them by 90 degrees, 
see the footnote on page 207. 

ei, the applied primary e.m.f., is equal and opposite to ei&. 

(6) Full-load conditions. Let the secondary circuit now be 
closed and let its resistance and reactance be such that i% lags 
e 2 by' a 2 degrees, then the voltage and current phase relations are 
as shown in Fig. 302. 

4> is the magnetic flux threading both coils and has practically 
the same value at full-load as at no-load. 




Fig. 301.— No load. Fig. 302.— Full load. 

Vector diagrams for a transformer. 

u is the component of the primary current required to produce 
the flux </>. 

ei is the applied primary e.m.f. 

e 2 is the secondary generated e.m.f. 

i 2 is the secondary current, whose value and whose phase angle 
a 2 depend on the constants of the connected circuit. 

in is the component of the primary current required to neu- 
tralize the demagnetizing effect of the current i 2 ; 

?ii?"n = n 2 i 2 

i\ is the primary current and is the resultant of U and in. 

If the current io is small, then «i = a 2 and n&i = n 2 i 2 . 

292. Induction Furnace. — An electric furnace which operates 
as a transformer is shown diagrammatically in Fig. 303 and is 
called an induction furnace, the secondary winding in this case 
being the charge which is contained in the annular channel A 



264 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxiv 

and is heated by the secondary current. The amount of energy 
put into the secondary can be varied by varying the applied 
primary voltage. 

Fig. 304 shows diagrammatically an electric welder which 
operates on the same principle. The single turn A in this case 
is open, and is closed by the two pieces to be welded, which 
pieces are held in the clamps B and are forced together under 
pressure while the welding current passes across the contact and 
heats the ends to be joined. 

It might seem that, since the secondary load in this case is a 
resistance load being the resistance of the molten metal, the 
power factor of the transformer would be high at full-load, and 
yet in practice it seldom exceeds 70 per cent. To explain the 




Fig. 303. — Induction furnace. 



Iron 
Core 



B B 

3 E 



Secondary 
"WinJiug 



V Primary 
Winding 



Fig. 304. — Induction welder. 



cause of this low power factor it is necessary to take up the subject 
of leakage flux in transformers. 

293. Leakage Reactance. — Fig. 306 shows the actual flux 
distribution in a transformer. The ampere-turns n\%i produce 
a flux 4>u, called the primary leakage flux, which is proportional 
to i\ and which threads the coil n\ but does not thread n 2 . 

The ampere-turns n 2 ii produce a flux <£ 2 /, called the secondary 
leakage flux, which is proportional to i 2 and which threads the 
coil n 2 but does not thread n\. 

The ampere-turns n\i\ and n 2 z 2 acting together produce a 
magnetic flux <f> which threads both coils n\ and n 2 and which is 
practically constant in magnitude; the effect of this constant flux 
has already been considered. 

Now any coil in which a current i produces a flux <j> which is 
proportional to the current is said to have self induction, see 



Art. 293] 



TRANSFORMER CHARACTERISTICS 



265 



Art. 239, page 206, and the voltage to send an alternating cur- 
rent / through such a coil = IX where X is the reactance of 
the coil; the current lags this voltage by 90 degrees, see page 
208. 

In Fig. 306, the flux <j>u is proportional to the current ii and 
its effect is the same as if the coil rii had a reactance X\ so that, 
instead of considering the effect of the flux 4>u, the effect of the 




Fig. 305. Fig. 306. Fig. 307. 

Fig. 305. — Ideal transformer. 

Fig. 306. — -Actual flux distribution in a transformer. 
Fig. 307. — Transformer showing the resistances and reactances diagram - 
matically. 

equivalent reactance X 1 may be considered. In the same way 
the leakage flux <p 2 i may be represented by an equivalent react- 
ance X 2 . Fig. 307 shows the diagram of an actual transformer 
in which the leakage fluxes <f)u and 4> 2 i are replaced by the equiva- 
lent reactances Xi and X 2 which, along with the resistances Ri 
and R 2 of the coils, are placed for convenience outside of the actual 
winding. 





Fig. 308. — Vector diagram Fig. 309. — Vector diagram 

of an ideal transformer. of an actual transformer. 



Between the terminals ab and cd, the transformer diagram in 
Fig. 307 is the same as the ideal diagram in Fig. 305. The vector 
diagram for this ideal transformer is shown in Fig. 308 which is 
the same as Fig. 302, page 263. 

The actual terminal voltage E t is obtained by subtracting from 
E 2 the vectors I 2 R 2 and /2X2, the voltages to overcome the sec- 



266 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxiv 



ondary resistance and reactance respectively, where I 2 R 2 is in 
phase with I 2 and I 2 lags I 2 X 2 by 90 degrees. 

The applied primary voltage E a is obtained by adding to Ex 
the vectors I1R1 and 7iXi, the primary resistance and reactance 
drops. 

In the elementary discussion of transformer operation, the 
resistance and reactance drops were neglected and it was found 



that 



E 2 
E~i 



n 2 



In Fig. 309 this relation still holds, but the ratio 



so that the transformer 



— is less than — - and so less than — , 

E a Ei fti 

ratio decreases as the load increases or the secondary voltage 
drops with increase of load; this drop however is small, being 
less than 2 per cent, for a 5 kv.a. transformer at 100 per cent, 
power factor, while for leading currents in the secondary circuit 
the secondary voltage may rise with increase of load just as in 
the case of the alternator, see page 245; the student may satisfy 
himself on this point by drawing the vector diagram. 

294. Leakage Reactance in Standard Transformers and in In- 
duction Furnaces. — In power transformers, the leakage reactances 



Primary 



Primary 




A B 

Fig. 310. — Leakage flux of transformers. 

Xi and X 2 are kept small by constructing the transformer so 
that 4>u and <£ 2 z are small. Diagram A, Fig. 310, shows a trans- 
former with a primary and a secondary coil on each leg and shows 
also the leakage fluxes. It may be seen from this diagram that, 
on each leg, 4>u an d 4>2i act in opposite directions, so that if n 2 
were interwound with wi then the leakage fluxes would neutralize 
and only the main flux <$> be left. This result is approximated in 
practice by constructing the transformer as shown in diagram B, 
where half of the primary and half of the secondary winding are 
placed over one another on each leg of the transformer core, the 
leakage fluxes have then to crowd into the space x between the 



Art. 295] 



TRANSFORMER CHARACTERISTICS 



267 



windings, and the smaller the space x, the smaller the leakage 
fluxes and the smaller the leakage reactances. 

In the induction furnace, unfortunately, the distance x cannot 
be made small, so that the reactances are comparatively large. 
The vector diagram for such a furnace is shown in Fig. 311. The 
secondary winding is short-circuited since it consists of a ring of 
molten metal, so that E t , the terminal voltage, is zero, and the 
secondary generated voltage E 2 is therefore made up of the two 
components I2R2 and J 2 X 2 , where R 2 is the resistance of the ring 
of molten metal and X 2 its reactance due to the leakage flux fci, 
Fig. 307. The remainder of this diagram is determined in the 
same way as in Fig. 309, and the power factor of the furnace is 
the cos a± and seldom exceeds 70 per cent. 




I,R 



Fig. 311. — Vector diagram for an induction furnace. 



295. The Constant-current Transformer. — For the opera- 
tion of arc lamps in series, the constant-current transformer 
shown in Fig. 312 is used. The primary coil is stationary and 
receives power at constant potential, while the secondary coil, 
which is suspended and is free to move toward or away from 
the primary, delivers a constant current to the lighting- 
circuit. 

When the secondary coil is close to the primary, the react- 
ances of the transformer are small and the secondary voltage 
is approximately equal to the primary voltage multiplied by 
the ratio of the number of turns. As the distance between 
the coils is increased, the leakage flux and the reactances 
increase and the secondary voltage drops, even although the 
primary voltage remains constant. 

The primary and the secondary currents are opposite in 
direction and, under these conditions, the primary and the 
secondary coils repel one another. The counterweight on the 
secondary coil is so adjusted that, when the desired current is 



268 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxrv 

flowing in this coil, the force of repulsion keeps the secondary 
coil suspended. If then some of the lamps in the circuit are 
cut out, the current tends to increase, but any increase in the 
current separates the coils and the voltage drops due to the 
increased reactance and so is unable to maintain the current 
at the increased value. The power factor of such a transformer 




Fig. 312. — Constant-current transformer. 

is high when the coils are close together but decreases as the 
distance between the coils is increased. 
296. The efficiency of a transformer 

output _ output 

input output + losses 

where the losses are : 

Iron losses; the hysteresis and eddy current losses in the core. 
Copper losses; these are Ii 2 Ri and 7 2 2 i? 2 watts respectively for 



Art. 300] 



TRANSFORMER CHARACTERISTICS 



269 




O 



the primary and secondary windings. There is no power loss due 
to the primary and secondary reactances, see page 209. 

297. Hysteresis Loss. — Since the flux in a transformer core 
is alternating, power is required to continually reverse the mole- 
cular magnets of the iron, this power is called the hysteresis loss. 

298. Eddy Current Loss. — If the transformer core in Fig. 313 
is made of a solid block of iron, then the alternating flux </> thread- 
ing this core causes currents to flow as shown at A, in the same 
way as through a short-circuited secondary winding. Power is 
required to maintain these eddy currents 

which power is called the eddy current loss. 

To keep these eddy currents small, a high 
resistance is placed in their path. This is 
accomplished by laminating the core, as shown 
at B, the laminations being separated from one 
another by varnish. 

299. Iron Losses. — The hysteresis and the 
eddy current losses taken together constitute 
what is called the iron loss. Since the flux 
per pole is practically constant at all loads, 
see page 262, therefore the iron loss caused by 
this flux is also constant at all loads. This 
loss may readily be determined by operating 

the transformer at no-load with normal voltage and frequencj 7 ", 
the input under these conditions, measured by a wattmeter, is 
equal to the iron loss, the small copper loss due to the no-load 
current being neglected. 

300. The all-day efficiency is defined as the ratio of the total 
energy used by the customer to the total energy input to the trans- 
former, during twenty -four hours. This efficiency is of impor- 
tance in the case of lighting transformers, which are connected 
to the mains for twenty-four hours a day but which supply 
energy for about five hours a day, under such conditions the all- 

, ffi . r output x h 

eye ciency - out p ut x ^ _j_ j ron i oss x 24 -f- copper loss x h 

where h is the number of hours per day during which energy is 
taken from the transformer. 

In a 50-kv.a. 2200- to 220-volt transformer the iron loss is 300 watts, the 
primary resistance is 0.5 ohms and the secondary resistance is 0.005 ohms. 
Find 

a. The efficiency when the load is 50 kw. and the power factor 100 per 
cent. 



Fig. 313.— Trans- 
former core. 



270 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxxiv 

b. The efficiency when the load is 5 kw. and the power factor 100 per 
cent. 

c. The efficiency when the load is 50 kv.a. and the power factor 80 per 
cent . 

d. The all-day efficiency in the latter case if the load is constant and con- 
nected for 5 hours a day while the transformer is connected to the line for 
24 hours a day. 

a. 1 2 , the secondary current = 50000/220 = 227 amp. 

1 1, the primary current = 50000/2200 = 22.7 amp. approximately 

Copper loss = 227 2 X 0.005 + 22. 7 2 X 0.5 - 514 watts. 

Iron loss = 300 watts 

Total loss = 814 watts 

Output = 50,000 watts , 

Input = 50,814 watts 

Efficiency = 98.5 per cent. 

b. h = 5000/220 = 22.7 amp. and I x - 5000/2200 - 2.27 amp. approx. 

Copper loss = 22. 7 2 X 0.005 + 2.27 2 X 0.5 = 5.14 watts 

Iron loss = 300 watts 

Total loss = 305 watts 

Output = 5000 watts 

Input = 5305 watts 

Efficiency = 94.4 per cent. 

c. J 2 = 50000/220 = 227 amp. and h = 50000/2200 = 22.7 amp. approx. 

Copper loss - 227 2 X 0.005 + 22. 7 2 X 0.5 = 514 watts 

Iron loss = 300 watts 

Total loss = 814 watts 

Output - 40,000 watts 

Input = 40,814 watts 

Efficiency = 98 per cent. 

, . All ., ffl • 40000 X 5 

a. All-day efficiency = — - - 

J 40000 X 5 + 300 X 24 + 514 X 5 

= 95.5 per cent. 

301. Cooling of Transformers. — Transformers become heated 
up due to the losses. This heat must be dissipated and the 
temperature of the transformer windings kept below the value 
at which the insulation begins to deteriorate. Transformers with 
an output of less than 1 kv.a. can dissipate their heat by direct 
radiation, but it is usual to place all transformers up to 500 kv.a. 
into a steel tank, which is then filled with insulating oil above the 
level of the windings. The oil improves the insulation, and con- 
vection currents are set up in the oil by means of which the heat 
is carried from the surface of the transformer to the larger surface 
of the tank, from which it is dissipated to the surrounding air. 
Such a transformer is said to be self cooled. 

The losses in a transformer are proportional to the volume of 



Art. 301] 



TRANSFORMER CHARACTERISTICS 



271 



the transformer, while the radiating surface is equal to the super- 
ficial area, so that, as the size of the transformer increases, the 
losses increase more rapidly than the radiating surface; special 
arrangements must therefore be made for cooling transformers of 
large output. 

For outputs from 50 to 500 kv.a., corrugated tanks such as 
that in Fig. 314 are used, while for outputs from 500 to 2000 kv.a., 




Fig. 314.— Corrugated tank. Fig. 315.- 



-Tank with external cooling- 
pipes. 



more surface must be provided than can be obtained from a cor- 
rugated tank and the construction shown in Fig. 315 is used; for 
outputs greater than 2000 kv.a., other methods of cooling allow 
the use of a smaller and cheaper transformer. 

The water-cooled type is shown in Fig. 316; cold water is cir- 
culated through the water coil and takes the heat from the hot 
upper layers of the oil. About 2 1/2 lb. or a quarter of a gallon 
of water is required per minute per kilowatt loss in the transformer. 

The air-blast type of transformer is shown in Fig. 317. The 



272 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxrv 

transformer is well supplied with ducts so that the air can reach 
the points at which the heat is generated. This type of trans- 
former is lighter than the oil-filled type but cannot be satis- 
factorily insulated for voltages greater than 30,000 volts. One 
hundred and fifty cubic feet of air is required per minute per 
kilowatt loss. 





Fig. 316 . — Diagram- 
matic representation of a 
water-cooled transformer. 



Fig. 317. — Diagrammatic represen- 
tation of an air-blast transformer. 




Fig. 318. — Method of cooling a transformer by circulating the oil. 

The circulating oil type of transformer is shown in Fig. 318 
and may be used with advantage where only hard water is avail- 
able for cooling purposes. When such water is used in water- 
cooled transformers, salts are liable to deposit inside the cooling 
coils and throttle the supply of water. With the circulating 
oil method of cooling, these salts will deposit on the outside of the 
cooling coils. 



CHAPTER XXXV 

TRANSFORMER CONNECTIONS 

302. Lighting Transformers. — These are generally built to 
transform from 2200 to 110 volts, but both primary and secondary 
windings are divided as shown diagrammatically in Fig. 319, and 
these windings may be so connected that a standard transformer 
can operate on the high-tension side at either 2200 or 1100 volts 
and on the low-tension side at either 220 oi 110 volts. The differ- 
ent connections used are shown in Fig. 319. 







to 220 Volts to 220 Vodta 

3 Wire 





2200 Volts to 110 Volts 



to 220 Volts to 220 Volts 

3 Wire 



Fig. 319. — Standard connections for a lighting transformer. 



303. Connections to a Two-phase Line. — In Fig. 320, diagram 
A shows the method of transforming from high-voltage two-phase 
to low- voltage two-phase, for the operation of motors. 

Specify the transformers for a two-phase motor which delivers 50 h.p. at 
440 volts with an efficiency of 90 per cent, and a power factor of 88 per cent., 
the line voltage being 2200. 

18 273 



274 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxv 

motor output = 50 h.p. 

= 50 X 0.746 = 37.2 kw. 
motor input = 37.2/0.9 = 41.4 kw. 

= 41.4/0.88 = 47kv.a. 

therefore two transformers are required each of 23.5 kv.a. output. The 
secondary current = 23500/440 = 53 amp., the primary current, neglect- 
ing transformer losses = 23500/2200 = 10.7 amp. 

Diagram B shows the method of connecting a low-voltage 
single-phase load to a two-phase line; the load should be divided 
as equally as possible between the two phases. 

Diagram C shows the Scott connection used to transform 
from two-phase to three phase. The two transformers x and y 
are wound for the same primary voltage, namely that of the 
two-phase line, but the secondary voltage of y is only V3/2 or 





Two Phase Generator 



A-TwoThas 
Motor 



B- Single Phase 
Load 




D - Voltages in a 
cott Connection 



Fig. 320. — Connection of transformers to a two-phase line. 

0.86 times that of x. One end of the secondary of y is connected. 

to the middle point of x. 

If the secondary voltage of x is E volts then that of y is ^3E/2 

volts and, in diagram D, Fig. 320, 

the difference of potential between a and b = E volts 

the difference of potential between a and c = \/(ao) 2 + (oc) 2 

= E volts 
the difference of potential between b and c = E volts 

and the phase relations between these voltages is the same as in 

a delta-connected bank of transformers. 

A load which is balanced on the three-phase side is also balanced on the two- 
phase side. Fig. 321 shows the currents and voltages in a Scott connected 



Art. 303] 



TRANSFORMER CONNECTIONS 



275 



group which is used to transform from two-phase at E volts to three-phase 
at E volts. 

Since the three-phase load is balanced, therefore 7i,7 2 and 7 3 are all equal, 
and the kv.a. output on the three-phase side = 1.73EI. 

Now the magnetizing effect of the primary of a transformer is always 
equal to the magnetizing effect of the secondary, so that in transformer y 



n ly X I y = n 2u X 7 



and 



therefore 



E 

~ V3E/2 

I X V3/2 



In transformer x, the current in oa = 1 and that in ob also = 7, but these 
currents are out of phase by 120 degrees and are in opposite directions there- 



iW 



i x ft 



£} Turns" 



eWnxTuT-llS 

— v Jl^ 



tliy Turns ->■ , 

innruinnnrtfWtf$p 

niy Tutus'' 



I 3 =I 

>-£ 



1-2- 



V Y 




B 



Fig. 321. — Voltage and current relations in a Scott-connected bank of 

transformers. 



fore the resultant magnetizing effect is not equal to no x I but to n 2x I X \/3/2 
see diagram B 



now 



^ = - = 1 

n 2x E 

and 711x7* = n 2x I X \/3/2 

therefore I x = I X V3/2 = I y 



and the total two-phase kv.a. = 2£7 X = y/'SEI, the same as on the three- 
phase side. 

If a 50-h.p., 440-volt, three-phase motor is supplied from a 2200 V£>lt, 
two-phase line, find the current 7, Fig. 321, and also the currents I x and I y , 
if the efficiency of the motor is 90 per cent, and the power factor is 88 per 
cent. 



276 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxv 



motor output = 50 h.p. 

= 50 X 0.746 = 37.2 kw. 
= 37.2/0.9 = 41.4 kw. 
= 41.4/0.88 = 47 kv.a. 

1.73 X#X/, _ 001 

kv.a., Fig. 321 



motor input 



1000 
1.73 X440 XI 



kv.a. 



and 



1000 
I = 47,000/1.73 X 440 



62 amp. 



neglecting transformer losses, 2 X 2200 X !%*= 47 kv.a., 
and h = 47,000/2 X 2200 = 10.7 amp. 

304. Connections to a Three-phase Line. — In Fig. 332: 
Diagram A shows three transformers connected Y on both 
primary and secondary sides. If E\ is the voltage between the 



To Generator 






r 

A 



A 
A 



Fig. 322. — Connection of transformers to a three-phase line. 

primary lines and E 2 is that between the secondary lines then 

The primary voltage of each transformer = Ei/^S, see page 236. 

The secondary voltage of each transformer = E 2 /^S. 

The transformation ratio of each transformer = Ei/E 2 . 

Diagram B shows the transformers connected delta on both 
primary and secondary sides: 

The primary voltage of each transformer = E h see page 237. 

The secondary voltage of each transformer = E 2 . 

The transformation ratio of each transformer = E A /E 2 . 

Diagram C shows the transformers connected Y on the prim- 
ary side and delta on the secondary side : 

The primary voltage of each transformer = 2?i/V3. 

The secondary voltage of each transformer = E 2 . 



Art. 304] 



TRANSFORMER CONNECTIONS 



277 



The transformation ratio of each transformer = E 1 /y/SE 2 . 

In a delta connection, the voltage of any one phase at any 
instant is equal and opposite to the sum of the voltages in the 
other two phases, see page 233, so that if, in Fig. 323, one trans- 
former A of a bank of delta- connected transformers is discon- 
nected, the difference of potential between a and b is unchanged, 
being maintained by the transformers B and C in series, so that 
three-phase power can still be obtained from the lines a, b and c. 
This connection is called the V or open delta connection and is 
shown in diagram D, Fig. 322. 




b h 

Secondary * 



Primary 

Delta connection. 



£L_a 




Primary 

Open-delta connection. 

Fig. 323. — Comparison between the closed-delta and the . open-delta 

connections. 



In the open delta connection in Fig. 323. 

The current in each transformer = //. 
The voltage of each transformer = E t 

F J 
The rating of each transformer = 1 nnr > kv.a. 

1 T\F T 

But the total load on the three-phase line = inno kv.a., 

page 237, so that the rating of each transformer is greater than 
half the total load by the ratio 2/1.73 or by 15 per cent. 



If 30 kv.a. has to be transmitted, then three delta-connected transformers 
may be used each with a capacity of 10 kv.a. 



278 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxv 

If one of these transformers is burnt out, then the power that can be trans- 
mitted by the remaining two transformers is not 20 kv.a., but 

= 20 X V3/2 = 17.3 kv.a. 

305. Advantages and Disadvantages of the Y and Delta Con- 
nection. — For a given voltage of 110,000 volts between lines, 
each transformer in a Y-connected bank has to withstand 
110,000/V3 or 64,000 volts and is therefore less liable to break 
down than the transformers in a delta-connected bank which 
have to withstand the full 110,000 volts. 

When one transformer in a Y-connected bank breaks down, the 
system can no longer operate three-phase, whereas, if the trans- 
formers were connected delta, the faulty transformer could be 



15 
Amp.' 



15 
Amp.' 



55 
Amp. 



LauJ LafilJ UajJ 



110 „_ 





A.- Connection Diagram 

Fig. 324. 



B- Vector "Diagrar 



disconnected and about 58 per cent, of the load could still be 
carried by the two remaining transformers connected open delta. 
Delta- connected, transformers are used for practically all low 
voltage distribution. The Y to delta connection is often used 
for high- voltage work, the transformers being Y connected on the 
high-voltage side. 



The load on a 2200-volt three-phase line consists of 1800, 1/2 amp., 110 
volt lamps on three circuits, and 200 h.p. of three-phase motors with an 
average power factor of 80 per cent, and an average efficiency of 88 per cent, 
all on one circuit. Draw the diagram of connections, specify the trans- 
formers, and find the current in the mains and also the resultant power 
factor. The connection diagram is shown in Fig. 324. 



Art. 307] TRANSFORMER CONNECTIONS 279 

The kv.a. output of each motor transformer 
= l/3(motor kv.a.) 
= 1/3(200 X 0.746 X Q j g X ^g) 
= 70 kv.a. 

«. t + * +u 3 X 70 X 1000 

the line current for the motors = i 79 v 99D0 = amp. 

the kv.a. output of each lighting transformer 

600 X 0.5 X 110 

1000 

= 33 kv.a. 

33,000 ,„ . L A1 

the primary current in each transformer = 9000" = amp., but these 

transformers form a delta- connected load on the line, see Fig. 269, page 237, 
therefore the current in each line = 1.73 X 15 = 26 amp. 
the resultant current in the line is the resultant of 26 amp. at 100 per cent, 
power factor and of 55 amp. at 80 per cent, power factor and is equal to 

/ = Va6 2 + be 2 , diagram B 

= V(0.8 X 55 + 26) 2 + (0.6 X 55) 2 

= 77.5 amp 

ab 0.8 X 55 + 26 
the power factor = — = ==-= 

CLC 4 1 .0 

= 90.5 per cent. 

306. Types of Transformer. — The transformer shown in Fig. 
325 is said to be of the core type. If the coil on limb B is placed 
on A, and the iron of B is split and bent over as shown in Fig. 326, 
the resulting transformer is said to be of the shell type. 

For three-phase transmission there is a considerable saving in 
cost and floor space if, instead of a bank of three transformers 
each in its own tank, a three-phase transformer is used in which 
the windings of the three phases are all placed on the same core 
as shown in Figs. 327 and 328. The principal objection to the 
three-phase transformer is that a breakdown in the winding of 
one phase puts the whole transformer out of commission, so that 
such transformers are used only in large central stations where 
there is ample reserve capacity. 

307. The Autotransformer. — If a reactance coil is placed across 
an alternating-current line as shown in Fig. 329, the voltage E 2 
obtained by tapping the coil as shown may have any value less 

than Ei, and, as in the transformer, ^ = = T . Since the 

JI12 ni ii 



280 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxv 

primary and secondary currents oppose one another, the current 
in the section ab is the difference between the currents 7i and 7 2 . 







Iron Core 










Coil 

A 


||!? 


1 



















/ 


mm 


\ 


/ 


m£ 


\ 




==P 






^= 




V 




J 


\ 




> 



Fig. 325. — Core type of trans- 
former. 







Coils 
k and B 


1 


I 





1 


r 


§§§§p 


> 


m 



Fig. 326.— Shell type of trans- 
former. 



ZL 



MIHIIM!— 



Fig. 327. — Three-phase core 
type of transformer. 



Core 



ML _ .Primary 

{^--Secondary 



J 



Fig. 328.— Three-phase shell 
type of transformer. 




no Turns 
b 



E-2 



Fig. 329. — Autotransformer. 



When the transformation ratio is comparatively small, the 
autotransformer, as this reactance coil is called, is cheaper than 



Art. 308] 



TRANSFORMER CONNECTIONS 



281 



the equivalent transformer with separate windings. It is much 
used for reducing the voltage applied to alternating-current 
motors during the starting period, see page 301. 

308. Boosting Transformers and Feeder Regulators. — The 
voltage of a line may be raised by a small amount if a standard 
transformer is connected as shown in Fig. 330. The voltage of 
the line is boosted by the amount of the secondary voltage E 2 - 



Power House 



Ei 



Feeder 
E1+E2 



*c_jh_ 



Fig. 330. — Boosting transformer. 



When the secondary side of the transformer is tapped so that 
the boosting effect can be adjusted, the resulting piece of appara- 
tus is the Stillwell feeder regulator. It is used to raise the feeder 
voltage above that of the power house, so as to compensate for 
the voltage drop in the feeder and maintain the voltage at the 
load. 

Another type of feeder regulator is shown in Fig. 331. The 




Induction regulator. 



secondary coil in this case is movable relative to the primary. 
When the coils are in the relative position shown in diagram A , 
the voltage E 2 has its maximum value; when the secondary coil 
is moved through 90 degrees relative to the primary, as shown in 
diagram B, then the flux 4> which threads the secondary coil is 
zero, and no voltage is induced in that coil. Such regulators may 
be made automatic if the core is turned by means of a small motor 



282 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxv 

which is controlled by a solenoid connected across the line to be 
regulated. When the line voltage increases, the plunger of the 
solenoid is raised and closes the motor circuit, and the motor 
then turns the regulator in such a direction as to lower the volt- 
age. When the line voltage decreases, the plunger of the solenoid 
drops and reverses the motor, so that it now turns the regulator 
in the opposite direction. 



CHAPTER XXXVI 
POLYPHASE INDUCTION MOTORS 

309. The induction motor is used for practically all the power 
work when only alternating current is available. The essential 




j®u 




Fig. 332. — Squirrel-cage induction motor. 



parts of such a machine are shown in Fig. 332. The stator or 
stationary part is exactly the same as that of an alternator, the 




Fig. 333. — Squirrel-cage rotor. 

rotor, however, is entirely different and the type most generally 
used, called the squirrel-cage type, consists of a cylindrical core 

283 



284 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxvi 



which carries a large number of copper bars on its periphery 
which bars are all joined together at the ends by copper or brass- 
end connectors as shown in Fig. 333. The action of this machine 
will be explained in detail. 

310. The Revolving Field. — Diagram P, Fig. 334, shows the 
essential parts of a two-pole, two-phase induction motor. The 
stator carries two windings M and N which are spaced 90 elec- 
trical degrees apart and, in the actual machine, are bent back so 
that the rotor may readily be inserted. These windings are 



Ph-l ph-2 




S, F, 





Fig. 



A 




B 




C 


1 


) 


I 1 - Im 
1^0 




1^0 
Ii = Im 


R 


I\ = -Im 
12 = 


l\ 

12 = 


= o 

-■ -Im 


334.— 


-Revolving 


field of a two-pole 


, two-phase 


induction motor 



connected by wires to two-phase mains and the currents which 
flow at any instant in the coils M and N are given by the curves 
in diagram Q ; at instant A for example the current in phase 1 = 
+ I m while that in phase 2 is zero. 

The windings of each phase are marked S and F at the terminals 
and these letters stand for start and finish respectively; a + 
current is one that goes in at S and a — current one that goes in 
atF. 

The resultant magnetic field produced by the windings M and 
N at instants A, B, C and D is shown in diagram R from which it 
may be seen that, although the windings are stationary, a revolv- 
ing field is produced which is of constant strength and which 



Art. 311] 



POLYPHASE INDUCTION MOTORS 



285 



goes through one revolution while the current in one phase passes 
through one cycle. 

311. The Revolving Field of a Three-phase Motor.— Diagram 
P, Fig. 335, shows the winding for a two-pole three-phase motor; 
M, N and Q, the windings of the three phases, are spaced 120 
electrical degrees apart. These windings are connected to the 
three-phase mains and may be connected either Y or delta, see 
page 237, in either case the currents which flow at any instant in 




A 


B 




c 


D 


ii = i m 


Z]= Vilm 




Ixl-Vilm 


I\ =-i"m 


i 2 =-v 2 i m 


h=VzIm 




!•>= Im 


Ii =V 2 Im 


I^-Vlhn 


I3 = - hn 


s 


I-i—VzIm 


I^ViIm 



Fig. 335. — Revolving field of a two-pole, three-phase induction motor. 

the coils M, N and Q are given by the curves in diagram R; at 
instant A for example the current in phase 1 = -\-I m that in phase 

2 = — y and that in phase 3 is also = — -—• 

The resultant magnetic field produced by the windings at 
instants A, B, C and D is shown in diagram S from which it may 
be seen that, just as in the case of the two-phase machine, a 
revolving field is produced which is of constant strength and 
which goes through one revolution while the current in one phase 
passes through one cycle. 



286 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap. 

t 1 



XXXVI 





A B 

Ii=Im I x -0 

Fig. 336. — Revolving field of an eight-pole, two-phase induction motor 



Art. 313] 



POLYPHASE INDUCTION MOTORS 



287 



312. Multipolar Machines. — Fig. 336 shows part of the wind- 
ing of an eight-pole, two-phase induction motor and also the re- 
sultant magnetic field at the instants A and B, Fig. 334. The 
field moves through the angle 6, or through half the distance 

between two adjacent poles or ~- of a revolution, in the time 

interval between the instants A and B or in j, seconds, therefore 

the speed of the revolving field 

= o~ X 4/ rev. per sec. 

120/ 

= rev per. mm. 

V 

This is called the synchronous speed, and is the speed at which 

an alternator with the same number of poles must be run in order 

to give the same frequency as that applied to the motor, the table 

on page 195 therefore applies to induction motors as well as to 

alternators. 

313. The Starting Torque. — Let the revolving field of a two- 
pole machine, produced by either a two- or a three-phase stator, 




Fig. 337.— Direction 
of the e.m.f. in the 
rotor bars. 



Fig. 338.— Direction 
of the currents in the 
bars of a low resistance 
rotor. 



Fig. 339.— Direction 
of the currents in the 
bars of a high resistance 
rotor. 



be represented by a revolving north and south pole as shown in 
Fig. 337. The field is moving in the direction of the arrow and 
therefore cuts the stationary rotor bars and generates in them 
e.m.fs. which are shown by crosses and dots. The e.m.f. will be 
a maximum in the conductor which is in the strongest field and the 
direction of the e.m.f. in each conductor may be found by the right 
hand rule, see page 192. Since the rotor winding forms a closed 



288 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxvi 

circuit, the generated e.m.fs. will cause currents to flow in the 
rotor bars. 

The frequency of the e.m.fs. generated in the stationary rotor 

bars by the revolving field = ^ ' ' P ' — : , see page 195, which 

is the same as the frequency of the e.m.f. applied to the stator, 

f X 120 
since the syn. speed = , see page 287, so that the fre- 
quency of the rotor currents is high, and the reactance of 
the rotor, which is proportional to the rotor frequency, is 
large compared with the rotor resistance, the rotor current in 
each bar therefore lags considerably behind the e.m.f. in that bar. 
In conductor a for example, the e.m.f. has just reached its maxi- 
mum value, but the current in that conductor lags the e.m.f. by 
an angle 6 which is almost 90 degrees and so does not become a 
maximum until the poles have moved into the position shown in 
Fig. 338, which is almost 90 degrees from the position shown in 
Fig. 337. 

Since the conductors in Fig. 338 are carrying current and are in 
a magnetic field, they are acted on by forces, the direction of which 
may be determined by the left-hand rule, from which it is found 
that while the force in the belts be and de tends to make the rotor 
follow the revolving field, that on the conductors in the belts cd 
and be acts in the opposite direction. The former force is the 
larger, so that the rotor tends to follow the revolving field. 

The current in the rotor conductors at standstill 

rotor voltage at standstill 
rotor impedence at standstill 

and the rotor impedence is large enough to limit the current to 
about five times the full-load value. On account of the large 
torque opposing the starting of the rotor, this large starting cur- 
rent produces an effective starting torque which seldom exceeds 
one and a half times the full-load torque of the motor. 

314. The Wound Rotor Motor. — The starting torque for a 
given current can be increased if that current is brought more in 
phase with the e.m.f. This may be seen from Fig. 339 in which 
the current lags by such a small angle 0that there is practi- 
cally no opposing torque. 

The angle of lag in a circuit may be decreased by increasing the 
resistance of the circuit without changing the value of the react- 



Art. 314] 



POLYPHASE INDUCTION MOTORS 



289 



ance. In Fig. 340 for example, when the resistance R is in- 
creased, the vector diagram changes from A to B; the current 1 
is decreased and so also is the angle 6. If then sufficient resist- 






->k — ix— > 




A B 

Fig. 340.— Effect of resistance on the magnitude and on the phase angle of 
current in a series circuit. 

ance is put into the rotor bars to reduce the starting current to 
the full-load value, the angle of lag of the rotor currents will be so 
small that the torque is all effective, and full-load torque is 



Ci 






L 



Slip Eiiig 



Fig. 341.— Squirrel-cage rotor with bars that have an adjustable resistance. 




Fig. 342.— Wound rotor. 

developed with full-load current. By using a lower resistance, 
twice full-load torque may be obtained with about twice full- 
load current. 

19 



290 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxvj 

When a motor is running under load, a large rotor resistance is 
undesirable because it causes large copper loss, low efficiency and 
excessive heating, so that some method must be devised whereby 
resistance can be inserted in the rotor conductors during the start- 
ing period and cut out during the running period. 

' This result could be attained by leaving the rotor bars open at 
one end and connecting that end to a slip ring on the shaft, 
as shown in Fig. 341, between which ring and the stationary 
end connector C an adjustable resistance R could be inserted. 
This construction would necessitate the use of as many slip 
rings as there are rotor bars so that it is modified in practice to 
reduce the number of slip rings. 

The bars are connected so as to forma winding of three groups, 
as shown diagrammatically in Fig. 342. One end of each group 
is connected to the end connector d, while the free ends are 
connected to slip rings and then through adjustable resistances 
Ri to the other end connector C, which is now merely a short 
piece of wire connecting the resistances together. The resistances 
.Ri are gradually cut out as the motor comes up to speed, and are 
finally short-circuited. This type of motor is called the wound 
rotor type of induction motor and is to be preferred to the squirrel 
cage type for heavy starting duty. 

315. Running Conditions. — It was shown on page 288 that 
the resultant starting torque is in such a direction as to make 
the rotor follow up the revolving field. When the motor is not 
carrying any load, the rotor will revolve at practically the speed 
of the revolving field, that is at synchronous speed; it can never 
run at a speed greater than that of the revolving field. If the 
motor is then loaded, it will slow down and slip through the 
revolving field, the rotor bars will cut lines of force, the e. m. fs. 
generated in these bars will cause currents to flow in them and a 
torque will be produced. The rotor will slow down until the point 
is reached at which the torque developed by the rotor is sufficient 
to overcome the retarding torque of the load. 

. syn. r.p.m. — r.p.m. of rotor .,,,,, 

The ratio is called the per cent. 

syn. r.p.m. 

slip, and is represented by the symbol s, its value at full-load is 

generally about 4 per cent., that is, when the revolving field 

make one revolution, the rotor makes 0.96 of a revolution and 

the field moves relative to the rotor bars through 0.04 of a 

revolution. 



Art. 316] 



POLYPHASE INDUCTION MOTORS 



291 



When the rotor is at standstill, the rotor frequency is / cycles 
per second, see page 288, where / is the frequency of the e.m.f . 
applied to the stator, but at full-load the rotor frequency is only 
sf cycles per second because then the velocity of the field relative 
to the rotor is only s per cent, of the relative velocity at standstill. 

As the load on the motor increases, the motor slows down and 
the rotor slips more rapidly through the revolving field and causes 
the rotor current to increase, but the frequency of the rotor cur- 
rent increases as well as its numerical value and this causes the 
rotor reactance to increase and the current to lag. Now the 
torque developed by the rotor tends to increase due to the increase 
in the rotor current and to decrease due to the increase in the 




200 400 600 800 1000 1200 1400 
K-.P.M. 



Starting current = 160 amp. = 5 times full load current 
Starting torque = 1.9 times full load torque 
Maximum torque =3.1 times full load torque 

Fig. 343. — The relation between torque, current and speed in a 25 h.p., 
440 volt, 3-phase, 60 cj T cle induction motor. 

current lag, see page 288. Up to a certain point, called the break- 
down point or point of maximum torque, the effect of the current 
is greater than that of the current lag, beyond that point the 
effect of the current lag is the greater, so that, after the break- 
down point is passed, the torque actually decreases even although 
the current is increasing. The relation between speed, torque and 
current is shown in Fig. 343 for a 25-h.p., 440 volt, three-phase, 
60-cycle, 1200 syn. r.p.m induction motor. 

316. Vector Diagrams for the Induction Motor. — a. No-load 
conditions. Alternating e.m.f s. E h applied to the stator, cause 
alternating currents I to flow in the stator windings and pro- 
duce the revolving field 0, but no work is done if the no-load 
losses are neglected, so that the current flowing in each phase 
must lag the voltage applied to that phase by 90 degrees, as 



292 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxvi 

shown in Fig. 344. This no load current is kept as small as pos- 
sible by the use of a small air gap clearance between the stator 
and rotor; a 50-h.p., 900-r.p.m. induction motor will have a rotor 
diameter of 20 in. (51 cm.) and an air gap clearance of 0.03 
in. (0.075 cm.) and, in spite of this small air gap, the magnetizing 
current 7 will not be less than 30 per cent, of full-load current. 
The revolving field, in addition to cutting the rotor conductors, 
cuts also those of the stator and generates an e.m.f . E ib , called the 



A^i 



ae 




Fig. 344.— No-load Fig. 345.— Ideal. Fig. 346.— Actual, 
vector diagram. Full-load vector diagrams. 



IUU 






A „. 


____-^ 














^"" 








80 
















/ 
/ 


B^ 




































£ 60 




(JT 


















































> 




























e-i 








o 






g 40 






• -J 
































Q 








fc 




















Ph 














20 














/; 
i 













1*4 iH 

Full Load 



Fig. 347. — The power factor of an induction motor. 

back e.m.f., in each phase of the stator winding. E lb is less 
than Ei by the e.m.f. required to send the magnetizing current 
7 through the impedence of the stator winding, which e.m.f. is 
comparatively small. The revolving field is therefore propor- 
tional to the voltage Ei. 

b. Full-load conditions. When the motor is loaded it slows 
down and current passes through the rotor windings and develops 



Art. 317] POLYPHASE INDUCTION MOTORS 293 

the driving torque of the machine. This current, like that in the 
secondary of a transformer, tends to demagnetize the machine or 
to oppose the flux which produces it, but a reduction in the 
flux causes the back e.m.f. E ib to decrease and allow a larger 
current to flow in the primary or stator winding, so that the 
stator current always adjusts itself to suit the requirements of 
the secondary or rotor. 

The reduction in the value of <f> is comparatively small, so that 
at full-load, a magnetizing component of current 7 is still required 
to produce the revolving field while a power component I w in 
phase with the applied voltage is required for the load, as shown in 
Fig. 345. The power factor of an induction motor, namely, the 
cos a, is therefore practically zero at no-load and gradually 
increases as the load increases, as shown in curve A, Fig. 347. 

Due to the reactance of the stator winding, and to that of the 
rotor winding which, as pointed out on page 291, increases with 
the load, the current lags more than shown in Fig. 345 and the 
power factor in an actual machine changes with the load as shown 
in curve B. 

317. Adjustable Speed Operation. — The squirrel cage motor is 
essentially a constant speed machine. For adjustable speed 
operation the wound rotor motor must be used. If such a motor 
is operating with a load having a constant torque, and a resistance 
is inserted in the rotor circuit, the rotor current will decrease and 
the motor will not be able to develop the necessary torque and so 
will slow down. As the speed drops however the rotor slips more 
rapidly through the revolving field, so that a greater e.m.f. is 
induced in the rotor conductors and the current increases, and, 
at some lower speed, is again sufficient for the load. 

The speed regulation however is very poor because, if the load 
were decreased, less current would be required, and the motor 
would automatically speed up so as to reduce the slip and 
thereby reduce the rotor voltage and current. At no-load, 
the rotor current being then very small, the slip would be 
practically zero and the motor would run at practically syn- 
chronous speed. This method of speed control is therefore 
similar to that of a direct- current shunt motor with resist- 
ance in the armature circuit and has the objection that the speed 
regulation is poor and the efficiency is low. 

Where good speed regulation is desired, the stator may be sup- 
plied with two separate windings, one of which is wound so as to 



294 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxvi 

produce a revolving field with p poles and the other a revolving 
field with pi poles and then, by using one or other of these wind- 

120/ 120/ 
ings, the synchronous speed may be or . It is sel- 
dom possible on account of expense to put more than two such 
windings on one stator. 

318. Induction Generator. — Suppose that an electric car driven 
by an induction motor is operating on a road where there is a 
long pull up followed by a long coast down. When on the up- 
grade, the motor runs at about 4 per cent, less than synchronous 
speed, at which speed the rotor slips fast enough through the 
revolving field to cause full-load current to flow. 

Starting on the down grade, the car begins to drive the motor 
and the speed of the motor first becomes equal to the synchronous 
speed, when the rotor current is zero, and then runs at a speed 
which is greater than synchronous so that the rotor is again slip- 




Fig. 348. — The rotor of a self -starting synchronous motor. 

ping through the revolving field. But since it is now running 
faster than the field, the direction of motion of the conductors 
relative to the field has been reversed, and the torque which was 
a driving torque now becomes a retarding torque so that the 
machine is now acting as a generator and delivering power to the 
mains. When the speed is about 4 per cent, above synchronous 
speed, the machine will be delivering full-load as a generator. 

319. Self-starting Synchronous Motors. — Polyphase syn- 
chronous motors have stators which are exactly the same as those 
of induction motors, they may therefore be made self starting if a 
squirrel cage winding is added to the rotor as shown in Fig. 348. 



Art. 320] POLYPHASE INDUCTION MOTORS 295 

If alternating e.m.fs. are applied to the stator of such a machine, 
the field coils not being excited, the machine will start up as an 
induction motor and will attain practically sjmchronous speed. 
If the field coils are then excited, the machine will be pulled into 
synchronism. 

The single-phase synchronous motor cannot be made self 
starting in this way because, as shown on page 308, the single- 
phase induction motor is not self starting. 

320. Dampers for Synchronous Machines. — In addition to 
making a synchronous machine self starting, the squirrel cage 
acts as a damper to prevent hunting, see page 259. 

A machine which is hunting is moving at a speed which is 
alternately slower and faster than the synchronous speed, and, 
in each case, the squirrel cage rotor cuts lines of force. A driving 
torque is produced as in an induction motor when the machine is 
running below synchronous speed, and this tends to speed it up, 
while a retarding torque is produced as in an induction generator 
when the machine is running above synchronous speed, and this 
tends to slow it clown. When the machine is running at syn- 
chronous speed there is no current in the squirrel cage. The tor- 
que due to the squirrel cage therefore tends at all times to pre- 
vent oscillations in speed and therefore to prevent hunting. 



CHAPTER XXXVII 
INDUCTION MOTOR APPLICATIONS AND CONTROL 

321. Choice of Type of Motor. — The synchronous motor is 

the only motor whose power factor can be controlled. Such a 
machine may be overexcited and made to draw a leading current 
from the line so as to raise the average power factor of the total 
load connected to the line and thereby allow the use of a smaller 
alternator and a smaller cross section of copper in the transmis- 
sion line than would be required for a load consisting entirely of 
induction motors, see page 257. 

The synchronous motor runs at a constant speed at all loads. 
It suffers from two disadvantages namely, that the starting 
torque is small even if the motor is of the self-starting type, while 
direct current is necessary to excite the field magnets. Because 
of these disadvantages it is used only in large sizes and only when 
the starting torque required is small as, for example, for the 
driving of large constant speed fans, pumps, and air compressors, 
when the load can be relieved during the starting period; it is 
also much used as the motor end of a motor generator set, see 
page 318. 

The single -phase machines described in the next chapter are 
more expensive than polyphase machines of the same output and 
are not used for general power work. They are used in compara- 
tively small sizes where polyphase current is not available. 

The polyphase induction motor is used for practically all the 
power work when only alternating current is available, care must 
be taken however to use the proper type. 

The squirrel-cage induction motor takes a lagging current and 
has a full-load power factor of about 80 per cent, for a 1 h.p. 
motor and 90 per cent, for a 100 h.p. machine. 

It is available only with the following speeds 

296 



Art. 324] 


INDUCTION MOTOR APPLI 


CATIONS 


Poles 


On 25 -cycle 


On 60-cycle 




mains 


mains 


2 


1500 r.p.m. 


3600 r.p.m. 


4 


750 r.p.m. 


1800 r.p.m. 


6 


500 r.p.m. 


1200 r.p.m. 


8 


375 r.p.m. 


900 r.p.m. 


10 


300 r.p.m. 


720 r.p.m. 


P 


120, X 25 r.p.m. 


120 X 60 r.p.m 



297 



The full-load speed is about 4 per cent, lower than the above. 

The principal objection to the squirrel-cage motor is that it takes 
a large starting current to develop a moderate starting torque. 
About 5 times full-load current is required to give 1.5 times full- 
load torque or, with reduced voltage, see page 302, 3.3 times full- 
load current for full-load torque. The motor is therefore not 
suitable for heavy starting duty. 

It is very rugged, has no sliding contacts, and is the most 
satisfactory constant speed motor for loads that do not require a 
large starting torque. 

The wound rotor induction motor has the same running char- 
acteristics as the squirrel-cage motor, but has better starting- 
characteristics and will develop full-load torque at starting with 
full-load current in the line. 

The service for which each of these induction motors is suited 
can best be illustrated by a few typical examples. 

322. A line shaft should run at practically constant speed at all 
loads and may be driven by a squirrel-cage motor if there are not 
many countershaft belts, or if loose pulleys are used to take the 
load off the motor during the starting period. If the starting 
torque required exceeds full-load torque then a wound rotor type 
of motor must be used. 

323. Wood-working machinery such as planers and saws are 
driven by squirrel- cage motors, except in the case of certain types 
of heavy planing mills which have a large inertia and require the 
large starting torque developed by the wound rotor motor. 

324. Cement Mills. — The squirrel-cage motor is used for the 
driving of cement machinery because it has no sliding contacts ; 
the commutator of a direct- current motor or the slip rings of a 
wound rotor motor would wear very rapidly in a cement mill. 

To obtain the high starting torque required for some of the 
machines, a large resistance must be inserted in the rotor circuit, 
see page 289. In the case of the squirrel cage motor this is done 



298 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxvii 

by making the rotor end connectors of high resistance metal. 
The resistance in this case remains permanently in the circuit 
and causes the efficiency at full-load to be about 5 per cent, lower 
than it would be with a wound rotor motor, since this latter ma- 
chine is so constructed that the resistance can be cut out when the 
motor is up to speed. 

325. Motors for Textile Machinery. — The quality of the 
finished product of a loom depends largely on the constancy 
of the motor speed and for such service induction motors are 
preferred to direct -current motors because their speed depends 
only on the frequency of the power supply and is not affected by 
variations of the applied voltage. 

326. Adjustable Speed Motors. — A wound rotor induction 
motor with the rotor short-circuited drops about 4 per cent, in 



Stator Switch 




abed 



100 

1 80 

D 

2 60 

A 
o 

» 40 

"o 

I 20 

3 







8 'fitch 


u CJosed 




^ 




^ 


i <£i %> £ 


^ 






V 




>\°. 


\" 








V 



















Fig. 349. 



25 50 75 100 125 150 

Pex Cent of Fall Load Toique 

Fig. 350. 



Effect of rotor resistance on the speed of an induction motor. 

speed between no-load and full-load, as shown in curve a, Fig. 350. 

When resistance is inserted in the rotor circuit, as shown in 
Fig. 349, the motor will drop in speed so as to generate the larger 
e.m.f. required to send the current through this higher resistance. 
At no-load however the rotor will run at practically synchronous 
speed because only a small slip is required to generate enough 
e.m.f. to send the no-load current through the rotor. Curves 
b and c, Fig. 350, are the speed curves with different values of 
rotor resistance; the larger the drop in speed required, the larger 
the rotor resistance must be, and the greater the loss in this 
resistance. 

327. Crane motors should be able to develop a large starting 
torque without taking an excessive current from the line; they 



Art. 329] INDUCTION MOTOR APPLICATIONS 299 

should run at a slow speed when the load to be lifted is heavy 
and at a high speed when the load is light. 

Large starting torque may be obtained by inserting resistance 
in the rotor circuit. By this means full-load torque may be 
obtained at starting with full-load current in the line, and twice 
full-load starting torque with about twice full-load current, see 
page 289. 

A drooping speed characteristic such as that in curve c, Fig. 350, 
is also obtained by inserting resistance in the rotor circuit. 

Crane motors in small sizes are generally of the squirrel cage 
type, the high rotor resistance being obtained by making the rotor 
end connectors of high resistance metal. This gives a very simple 
type of machine but, because of the large rotor resistance and 
therefore the large rotor resistance loss, the machine heats up 
quickly. For outputs greater than 20 h. p. it is therefore advisable 
to use the wound rotor type of machine in which case the 
resistance is outside of the machine and can be varied as desired. 

Crane operation by means of induction motors is not so efficient 
as the operation by direct-current series motors, since these latter 
machines have the desired characteristics without the use of 
additional resistance, see page 103. 

328. Shears and punch presses are generally supplied with a 
flywheel to carry the peak load. In order that the flywheel 
may be effective, the speed of the motor must drop as the load 
comes on. A standard machine drops in speed about 4 per cent, 
between no-load and full-load. For a larger drop in speed, 
resistance must be connected permanently in the rotor circuit 
so as to give a characteristic such as that shown in curve b, Fig. 
350. The motor may be a squirrel-cage machine with high 
resistance end connectors, but for large presses it will often be 
necessary to use a wound rotor motor to obtain sufficient starting 
torque to accelerate the flywheel on first starting up. 

329. For adjustable speed service such as the driving of lathes 
and other machine tools, the only alternating- current motor at 
present available is the wound rotor induction motor. 

The speed of such a machine may be lowered for a given torque 
by inserting resistance in the rotor circuit as shown in Fig. 349, 
and the speed characteristics with different values of resistance 
are shown in Fig. 350. 

Because of the use of this resistance, there is a large resistance 
loss which increases as the speed is decreased, so that the efficiency 



300 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxvii 

of an induction motor at reduced speed is low. Moreover the 
speed regulation is poor, as shown by curves b and c, Fig. 350. 
If for example an induction motor operating at reduced speed is 
driving a lathe in which a forging such as that in Fig. 129, page 108, 
is being turned, then the motor will slow down when the cut is 
deep and will speed up when the cut is light, so that the speed 
will be very irregular. 

For machine tool driving then the alternating- current motor 
is not such a suitable machine as the direct-current shunt motor 
controlled by field resistance, see Arts 126 and 127, page 107, 
and it has been pointed out that for crane service the direct- 
current motor is superior to the alternating- current motor, so 
that, when the bulk of the load consists of adjustable speed or 
crane motors, direct current should be supplied even if it is neces- 
sary to use a motor generator set, see page 318, to change from 
alternating to direct current. 

330. Resistance for Adjustable Speed Motors. — As in the case 
of the direct- current motor, the number of ohms in the controlling 
rheostat and the current carrying capacity of the resistors de- 
pends on the service for which the motor has to be used. 

If a fan and a reciprocating pump take 10 h.p. at full 
speed, then at half speed the pump takes 5 h.p. since the 
torque is constant, while the fan takes V 10 = 2. 15 h.p., see 
page 111, the rotor current of the fan motor will therefore 
be less at half speed than that of the pump motor. The voltage 
generated in the rotor by the revolving field, however, will be the 
same in each case, so that the number of ohms in the fan rheostat 
must be greater than the number in the pump rheostat so as to 
reduce the current to the lower value. 



STARTERS AND CONTROLLERS 

331. Switches for Alternating -current Circuits. — It causes 
less arcing to open a circuit in which alternating current is flowing 
than to open one in which direct- current is flowing, because, in 
the former case, the current passes through zero twice every cycle. 

The quick break type of switch, see page 114, is used for small 
currents. For larger currents or in high-voltage circuits the 
contacts are immersed in insulating oil which quenches any arc 
that forms. The oil switch in alternating- current circuits takes 



Art. 332] 



INDUCTION MOTOR APPLICATIONS 



301 



the place of the magnetic blow-out switches used in direct- current 
circuits. 

332. Starting of Squirrel-cage Induction Motors. — A squirrel 
cage motor at standstill takes about 5 times full-load current 
from the line with normal applied voltage, and develops about 
1.5 times full-load torque. 



Running Position 
Starting Positio 




Fig. 351. 



-Connections for a small three-phase, squirrel -cage induction 
motor. 



Motors with an output of 5 h.p. or less are generally 
connected directly to the line without a starter, by means of a 
double-throw switch such as that shown diagrammatically in 
Fig. 351. This switch is so constructed that it will stay in the 
starting position only when held there against the tension of a 
spring. 




Auto Transformers 

Fig. 352. — Connections of a starting compensator for a three-phase, 
squirrel-cage induction motor. 

Motors with an output which is greater than 5 h.p. are' 
generally started up on reduced voltage by means of autotrans- 
formers connected as shown in Fig. 352. By this means the start- 
ing current is reduced, but the starting torque also is reduced; 
the revolving field <j> is proportional to the applied voltage E which 
produces it, see page 292, while the rotor current 7 2 is proportional 
to the revolving field <f> by which it is produced, so that the start- 



302 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxvii 

ing torque, being proportional to 4> X 1 2 is proportional to </> 2 
and therefore to E 2 . 

A motor at standstill takes 5 times full-load current with normal applied 
voltage and develops 1.5 times full-load torque. What must the applied volt- 
age be to obtain full -load torque and what will be the starting currents in the 
motor winding and in the line ? 

( E2) 2 full-load torque 1 

(E x y ~ 1.5 full-load torque" = L5 

therefore E 2 = A / — X E x 



^l 



= 0.815^i 

or 81.5 per cent, of normal voltage 

E 2 
The starting current in the motor = 5 (full-load current) — 

Ei 

= 4.1 (full-load current) 
= I m , Fig. 352. 

The starting current in the line =4.1 (full-load current) — 

Ei 

= 3.3 (full- load current) 

= Ii, Fig. 352. 

333. Starting Compensator. — The combined autotransformers 
and switches constitute what is called a starting compensator. 
One type is shown in Fig. 353 and consists essentially of three 
autotransformers T and a double-throw switch S by means of 
which the motor is -connected to low- voltage taps for starting and 
is then connected directly to the line when nearly up to full speed. 

The complete connections of this compensator are shown in 
diagram A, Fig. 354. 

Diagram B shows the connections during the starting period; 
normal voltage Ei is applied to the lines a, b, and c while a reduced 
voltage E 2 is tapped off from the autotransformers and is applied 
to the motor. 

Diagram C shows the connections during the running period. 
The voltage applied to the motor is now normal and the overload 
relaj^s are connected in the circuit while the no- voltage release 
coil M is connected across one leg of the circuit. 

The no-voltage release feature is similar in principle to that 
used on starters for direct-current motors, see page 120, and 
consists of a latching solenoid which holds the starting arm against 
the tension of a spring. When the line voltage fails, the solenoid 



Art. 333] INDUCTION MOTOR APPLICATIONS 



303 




Fig. 353. — Starting compensator for a three-phase induction motor. 



From Generator 



Overload 
Eelay 



Uo Voltage 
Release 



L 



Overload 
Relay 



Stationary 
■ Contacts 



1 1 — Moving 
Contacts 



Stationary- 
Starting 
Contacts 




o o o o o c 

Auto Transformers 

A B C 

Complete connections Starting Running 

Fig. 354. — Diagram of connections of a three-phase starting compensator 



304 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxyii 

M is deenergized and the starting handle then returns to the 
off position. 

In the case of a heavy overload, the plungers of the overload 
relays are raised and open the circuit of the no- voltage release 
M, the starting handle then returns to the off position. 

The no- voltage release is not supplied in all cases because an 
induction motor can carry a starting current of 5 times full-load 
current without injury during the short interval of time taken to 
attain full speed. As a rule, the only objection to this large 
current is the disturbance it causes due to the opening of circuit 
breakers. 

334. The star-delta method of starting is sometimes used for 
three-phase motors. The windings of the three phases are kept 




' I s at Standstill 



\ 



VT 3 

"A- Starting Connection 



at Standstill 



at Standstill V"3 

B- Running Connection 

Fig. 355.— Voltage and current relations in a Y-delta starter. 



separate from one another and six leads are brought out from the 
machine. Under normal running conditions the windings are 
delta connected as shown in diagram B, Fig. 355, and the voltage 
per phase is E 1 volts. During the starting period the windings 
are Y-connected as shown in diagram A in which case the voltage 
per phase is equal to #1/1. 73 or 58 per cent, of normal voltage. 

If a delta-connected motor at standstill takes 5 times full-load current with 
normal applied voltage and develops 1.5 times full-load torque, what is the 
starting current in the motor and also in the line, if the motor is Y-connected, 
and what is the starting torque under these conditions ? (The student should 
make a diagram of connections showing the double-throw switch required to 
change from Y to delta.) 

The starting torque = 1.5 (full-load torque) X (0.58) 2 
= 0.5 (full-load torque) 



Art. 335] 



INDUCTION MOTOR APPLICATIONS 



305 



The starting current in the line when delta connected 
The starting current in the motor when delta connected 
The starting current in the motor when Y-connected 



= Is 

= L/y/z 

= (Is/VS) X 0.58 
= L/3 
The starting current in the line when Y-connected = 1.,/S 

Now I s is 5 times full -load current, so that, when the motor is Y-connected, 
the starting current in the line is 1/3 X 5 or 1.67 times full-load current. 
The starting torque, however, is only 0.5 times full-load torque and, if this 
is not sufficient to start the motor with the load, then a starting compensator 
will be required. 

335. Starter for a Wound Rotor Motor. — To start up a motor 
of this type the main switch S, Fig. 349, is closed and then the 



I 


■%^ 3=3 






* 1 






1 






Fig. 356. — Sliding contact type of starter for a wound-rotor induction motor . 



resistance R in the rotor circuit is gradually cut out as the motor 
comes up to speed. 

Since the rotor is wound in three sections, see page 290, three 
sets of contacts are required which for small motors are generally 
mounted on a faceplate as shown in Fig. 356. This starter may 
be used as a speed regulator if the resistance has sufficient 
current carrying capacity. 

For motors larger than about 50 h.p., the multiple switch 
type of starter, see page 119, is generally preferred. Such a 
starter for the motor shown diagrammatically in Fig. 357 would 
consist of three double-pole switches so interlocked that they can 
be closed only in the order A, B, C. The switches are held closed 

20 



306 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap. 



XXXVII 



by a latch which is released by the no-voltage release magnet when 
the voltage fails. 

336. Automatic Starters. — Squirrel-cage motors of small out- 
put are thrown directly on the line and the self starter for such a 
motor consists of a single double-pole magnetic switch such as 
that shown diagrammatically at A, Fig. 357. 

If a solenoid were attached to the handle of a starting rheostat 
such as that in Fig. 356, then a sliding contact type of self starter 





Fig. 357. — Wound rotor type of motor. 





r 
























a 


' 
















si 


3 


^m 


¥ 


^ 


J 




b 


i^ 






l r/i 






^^- 




H 


rr . - 




6 


ffl 


b 






I 


6 


■>\ c r f a i i i * -i 



a 4 



Fig. 359. 



Fig. 358. — Automatic starter for a wound-rotor in- 
duction motor. 



would be produced similar to the direct- current starter shown 
in Fig. 154, page 130. If in addition the main starter switch is 
magnetically operated then the motor may be started from a 
distance by a control circuit such as that shown in Fig. 156, page 
131, this circuit being connected across one of the phases and the 
magnets being laminated 1 so as to be suitable for operation with 
alternating currents. 

1 The core of an alternating- current magnet must be laminated for the same 
reason as a transformer core is laminated, see page 269, namely, to prevent 
excessive core loss due to the alternating magnetic flux. 



Art. 336.] INDUCTION MOTOR APPLICATIONS 307 

For large motors, the multiple switch type of starter is used, 
by means of which the switches A, B and C, Fig. 357 are closed 
in their proper order and the motor thereby brought up to speed 
without the starting current exceeding a predetermined value. 
The principle of operation is the same as for the direct-current 
starter described in Art. 158, page 133, although the mechanical 
details are different. 

Fig. 358 shows the control circuit for the three switches A, B 
and C in Fig. 357. When the control switch s is closed, the line 
b is excited as far as point c, and the magnet X, connected between 
points 1 and 2, closes the double contactor switch A. The 
motor then starts up with all the rotor resistance R, Fig. 357, 
in the circuit, while about one and a half times full-load current 
flows in theTines. 

When switch A closes, the disc d drops, closes the contacts e and 
/, and extends the excited part of line b as far as g. The coil t is 
now excited from the points 3 and 4 and its plunger is lifted, 
thereby tilting the lever j to which it is attached and removing 
the support of the plunger of coil u. The line current passing 
around u however holds up this plunger until the motor has 
attained about one-third of normal speed and the current has 
decreased to about full-load value when the plunger of u drops 
and closes the contacts h and k. The coil Y is now excited from 
the points 5 and 6 and closes the double-pole switch B thereby 
cutting out the first step of the resistance from each leg of the 
rheostat. The current then increases to about 1.5 times full-load 
current but gradually decreases as the motor speeds up further. 

As soon as switch B closes, the interlock discs attached to that 
switch are lowered, and the contact Im is closed, so that coil Y is 
now excited from the points 5 and 7 and the switch B is therefore 
held closed independently of the plunger of coil u. The contact 
np is closed at the same instant by the lower interlock disc and 
the excited part of line b is thereby extended as far as q, and the 
same series of operations is repeated before switch C closes. 

Fig. 359 shows the complete starter, with the relays t and v 
on the top panel and the switches A, B and C on the lower 
panels. 




CHAPTER XXXVIII 
SINGLE-PHASE MOTORS 

337. Single-phase Induction Motors. — If one of the phases of a 
two-phase induction motor is opened while the motor is running, 
the machine will continue to rotate and carry the load. 

A two-pole single-phase induction 
motor at standstill is shown diagram- 
matically in Fig. 260. When an al- 
ternating current flows in the winding 
A, an alternating flux is produced. 
Since this magnetic field is not rotat- 
ing, there is no tendency for the rotor 
to turn, so that the machine is not self 
starting. FlG . 360.— Diagrammatic 

338. Split -phase Method of Starting, representation of a single- 
~ - ,, , , , , , phase induction motor. 

— One of the many methods used to 

obtain a rotating field from a single-phase supply is shown dia - 
grammatically in Fig. 361. The motor is wound as for two- 
phase operation, and a resistance R is inserted in series with 
the winding A . The current I a therefore does not lag as much 
as the current lb, and the resultant magnetic field of the motor 
has a rotating component which will cause the rotor to turn. 

If, in addition to the resistance R, a condenser C is inserted in 
series with the winding A, then the current I a may be made to 
lead the voltage E. The currents I a and lb will then be more 
nearly 90 degrees out of phase, and the operation of the motor 
will be more nearly that of a two-phase machine. 

This method of starting is called the split-phase method. 

339. Running Torque of a Single-phase Motor. — If, as in 
Fig. 363, two equal vectors P rotate with the same velocity but in 
opposite directions, the resultant vector R alternates between the 
values R = 2P and R = —2P, and always lies in the line joining 
these two points. 

An alternating magnetic flux </>, such as that in Fig. 362, is 
therefore equivalent to two rotating fields <\> x and 4> y which are of 

308 



Art. 339] 



SINGLE-PHASE MOTORS 



309 



equal strength and rotate in opposite directions with the same 
velocity. These fields tend to start the rotor in opposite direc- 
tions, so that the resultant starting torque is zero. 






Resistance in series with one Resistance and condenser in 
winding series with one winding 

Fig. 361. — Split-phase connections for a single-phase induction motor. 





-2P 

Fig. 362. Fig. 363 

Figs. 362 and 363. — Resolution of an alternating field into two revolving 

components. 

Suppose that the rotor has been started by phase-splitting and 
is running in the direction of the field <j> x with a speed which is 
little less than synchronous speed. The rotor bars will then be 
cutting the field </> x , and an e.m.f. will be induced which will send 



310 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxvm 

through these bars a current I x of frequency sf, see page 291, 
where s is the per cent, slip and/ is the applied frequency. Since 
this rotor frequency is low, the current lags by a very small angle, 
and the torque due to this current is large, see page 288. 

The rotor bars, however, are also cutting the field <f> y , and, 
since this field is moving in a direction opposite to that of the 
rotor, the frequency of the resulting rotor current I y is almost 
equal to 2/. This frequency is high, and the current I y lags by 
a large angle; so that the torque due to this current is negligible, 
see page 288. 

The resultant rotor current is the resultant of I x and I y , but' 
the latter current is not effective in producing torque, and the 



Armature Flux (pa 




Fig. 364. Fig. 365. 

Figs. 364 and 365. — Single-phase series motor. 

machine runs as if it had only one field 4> x . The characteristics 
with respect to slip, efficiency and power factor are therefore 
similar to those of a polyphase induction motor. 

340. Single-phase Series Motor. — This machine is wound and 
connected like a direct- current series motor, but a few structural 
changes are necessary to make a machine which will operate with 
alternating current. 

In Fig. 364, the current is flowing through such a machine in 
one direction, while in Fig. 365, half a cycle later, the current 
is flowing in the opposite direction in both armature and field 
coils, but the direction of the torque is unchanged. 

The characteristics of this machine are similar to those of the 
direct- current series motor. The torque is approximately pro- 
portional to the square of the current, since torque = K<t>I, 
where the flux <j> is proportional to the current I. The torque, 
however, is pulsating, being a maximum when the current is a 
maximum and zero when the current is zero, but the average 



Art. 340] 



SINGLE-PHASE MOTORS 



311 



torque is the same as would be produced by a direct current of 
the same magnitude. 

Since the magnetic flux in the poles is alternating, the field 
structure must be laminated to keep the eddy current loss small. 
Due to this alternating flux, a voltage E f of self induction is 
induced in the field coils, to overcome which the applied voltage 
must have a component E f , Fig. 366. 





& Motor Current 

Fig. 366. — Vector diagram for a single-phase series motor. 

Since the armature is rotating in a magnetic field, a back e.m.f. 
is generated in the armature in the same way as in a direct- current 
motor, see page 79. This e.m.f., however, is alternating since 
it is produced by the cutting of an alternating flux <f>. It is a 
maximum when the flux is a maximum and zero when the flux is 
zero, so that, to overcome this back e.m.f., the applied voltage 

100 



SO 



60 



40 







-^^o 













































25 50 

Per Cent of Ful 



75 100 

Load Cui rent 



125 



Fig. 367. — Characteristics of a single-phase series motor. 

must have a component E a in phase with the flux (/> and therefore 
in phase with the current 7. 

In Fig. 366, then 

4> is the alternating magnetic flux 

I is the current in the machine 

E a is the component of voltage to overcome the back e.m.f. 



312 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxviii 



E f is the component of e.m.f . to send the current I through the 
self induction of the field coils 

E, the resultant of E a and E f , is the applied voltage 

cos a is the power factor of the motor. 

The lower the frequency of the supply, the smaller the value 
of E f ) and the higher the speed of the motor, the larger the 
value of E a , so that low frequency and high speed are the condi- 
tions for high power factor. 

If the load on such a motor is increased, the current increases 
and so therefore does the flux </> and the voltage E f . Since the 





Fig. 368. — Conductively compen- Fig. 369. — Inductively compen- 

sated, sated. 

Figs. 368 and 369. — Methods of compensating for armature reaction in a 
single-phase series motor. 

applied voltage E is constant, therefore E a decreases slightly; to 
generate this back e.m.f. with the larger flux <£, the armature 
must run at a slower speed. Typical curves for such a motor 
are shown in Fig. 367. The speed and torque curves are similar 
to those of a direct-current series motor, and such a machine will 
run equally well with either a direct- or an alternating-current 
supply. 

341. Armature Reaction. — From Figs. 364 and 365 it may be 
seen that there is a cross magnetizing effect due to the armature 
current just as in the direct-current machine, but this cross flux 
<j> a is alternating and generates an e.m.f. of self induction in the 
armature coils which tends to lower the power factor, because 
self induction always tends to make the current lag. This cross 
flux is eliminated by means of a compensating winding connected 
as shown in Fig. 368 so that its magnetizing effect at every instant 
is exactly equal and opposite to that of the armature. 



Art. 342J 



SINGLE-PHASE MOTORS 



313 



Another method of eliminating the cross flux <j> a is shown 
diagrammatically in Fig. 369. In this case the compensating 
coils are short circuited. The flux </> a threads these coils and 
generates in them an e.m.f. which, according to Lenz's law, sends 
a current in such a direction as to oppose the change of the flux, 
so that the cross flux is automatically kept down to a small value. 

342. The Repulsion Motor. — If, in the machine shown in Fig. 
368, the armature is disconnected from the circuit and is then 
short circuited, as shown in Fig. 370, the resulting machine is 
what is called the repulsion motor. 




XX 


ft /^~®^\ 115 


XX 


B 


6 < c 


x ®\\ 


B 


l\ 


\\<8 


, . t . 


a 




•• X ^J 


SL^ - 


' ' 




Figs. 



A 

Fig. 371. Fig. 372. 

370 to 372. — The starting torque of a single-phase repulsion motor. 



If the coils B alone are acting, as shown in Fig. 371, then 
e.m.fs. are generated in the armature coils in such a direction 
as to oppose the change of the flux (j>, but no current passes 
through the armature winding or through the conductor x since 
the voltage between a and b is opposed by that between b and c. 

If the coils A alone are acting, as shown in Fig. 372, then a 



314 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxviii 

large current can flow, but the resultant torque is zero because, 
under each pole, half of the conductors carry current in one direc- 
tion while the other half carry current in the opposite direction. 
If both sets of coils are acting, as in Fig. 370, then the current 
produced in the armature by coils A can produce a torque with 
the magnetic field produced by coils B. 

343. Commutation of Series and Repulsion Motors. — The 
coils which are short circuited by the brushes on the commutator 
are threaded by an alternating flux 0, see Figs. 364 and 370, so 
that large currents are induced in these coils which currents, 
flowing through the brush contacts, cause sparking. The com- 
mutation of these machines is therefore much poorer than that of 
the direct- current series motor. 

344. Wagner Single-phase Motor. — Because of the poor 
commutation, the single-phase repulsion motor has not come into 
extensive use in this country except in small sizes. The principle 
of^the repulsion motor, however, is used by the Wagner com- 
pany to obtain a large starting torque from a single-phase in- 
duction motor. 

This machine starts up as a repulsion motor and, when it has 
attained almost synchronous speed, a centrifugal device short 
circuits all the commutator bars on one another and at the same 
time lifts the brushes. The motor operates thereafter as a single- 
phase induction motor and the commutator has a long life because 
it is used only during the starting period. 



CHAPTER XXXIX 
MOTOR-GENERATOR SETS AND ROTARY CONVERTERS 

345. Motor-Generator Set. — When it is desired to change from 
one alternating voltage to another a static transformer is used, 
see Chap. 34. To change from one direct voltage to another 
no such simple device is available and a motor-generator set has 
to be used. This consists of two machines mechanically con- 
nected together one of which, running as a motor, takes power 
from the source of supply and drives the other machine as a gen- 
erator; this latter machine is wound for the desired voltage. 

If 10 kw. at 220 volts is required from a 110 volt line then: 
the generator output = 10 kw. at 220 volts 

the motor output = jar X cTqq = 15 horse-power at 110 volts 

where the generator efficiency is taken as 88 per cent. 

346. The booster set is a special type of motor-generator set 
and is shown diagrammatically in Fig. 373. The generator in 




Fig. 373. — Booster motor-generator set. 

this case is connected in series with the line and its voltage can 
add to or subtract from the line voltage. 

If 10 kw. at 220 volts is required from a 110 volt line and a booster set 
is used then: 
the booster output = EI, Fig.373 

= 5 kw. at 110 volts 

5 X 1000 1 

the output of the driving motor = — ^a — X TToR 

= 8 horse-power at 110 volts 
and this set can perform the same duty as that performed by the motor- 
generator set figured out in the last article. 

315 



316 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xxxix 

347. The balancer set is a special type of motor-generator set 
used when it is desired to change from a two-wire to a three -wire 
direct- current system. As shown diagrammatically in Fig. 374, 
it consists of two shunt- wound direct- current machines with their 
armatures on the same shaft, which armatures in the particular 
case shown, are wound for 110 volts. 

Compare now the operation of a three- wire system without a 
balancer as in Figs. 375 and 377, and with a balancer as in Figs. 




Fig. 374. — Balancer set. 




220 

I 



Jttl 



T 

110 

4 



^u 



nttr 



Fig. 375. Fig. 376. 

Figs. 375 and 376. — Balanced load. 



^T 



i 

220 



Q 6 O 73 @iioj/«ff a O COCA (M)noJi a R a A i A 

1_1 ^0 Ml I<l I 220 Y > I ■ r 

; (Gjno.-fuRD Q o o 



147 



11 



Fig. 377. Fig. 378. Fig. 379. 

Figs. 377 to 379. — Unbalanced load. 



376 and 378. In the case where the load is equal on the two 
sides of the system, there is no current in the neutral wire and 
the balancer in Fig. 376 is running light as two motors in series 
on 220' volts, while the voltages across the two sides of the 
system are equal. 

If now some of the lamps on the side B of the system are 
switched off then in Fig. 377 the same total current is passing 
through the lamps connected across A as is passing through the 
smaller number of lamps connected across B so that each lamp B 
is carrying more current than each lamp A and the voltage across 
B is now greater than that across A. In the extreme case when 



Art. 348] 



MOTOR-GENERATOR SETS 



317 



only one lamp is connected across B the voltage across that lamp 
is practically 220 volts. 

When the balancer is used, as in Fig. 378, the voltages tend to 
take the same values as in Fig. 377, but when the voltage of B 
rises and that of A falls then the machine M tries to increase in 
speed and G to decrease in speed or each machine tries to run as a 
motor at the speed corresponding to its voltage. But the two 
machines are coupled together, so that the actual speed must be 
between these two values and the one that tends to run fast will 
try to increase in speed and will therefore act as a motor and 
drive the other machine as a generator, so that G operates as a 
generator and current passes through it from the negative to the 
positive terminal while M operates as a motor driving G and 




Fig. 380. — Three-wire generator. 



current passes through it from the positive to the negative termi- 
nal. The voltage across G will be (110 — I a R a ) volts and that 
across M will be (110 + I a R a ) volts, where I a R a is the armature 
resistance drop, and the change in the voltages across the two 
sides of the system is comparatively small even when the load is 
considerably unbalanced. 

Neglecting the losses in the balancer itself, the generator out- 
put and the motor input are each equal to (110 volts X half the 
unbalanced current), and if care is taken to arrange that the load 
is nearly balanced under all conditions, then the balancer may 
be of comparatively small output. 

When the load on side B of the system becomes greater than 
that on side A , the current in the neutral wire flows in the opposite 
direction, as shown in Fig. 379. 

348. Three-wire Generator. — The balancer set is eliminated 
by the use of the three-wire generator shown diagrammatically 
in Fig. 380, which consists of a standard two-wire direct-current 



318 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxix 

generator with a coil C of high reactance and low resistance con- 
nected permanently across diametrically opposite points on the 
armature. The voltage between a and b is alternating, see page 
240, so that, even with no external load, an alternating current 
flows through the coil C, this current, however, is extremely 
small since the reactance of coil C is large. 

The center point o is always midway in potential between a 
and b and this point is connected to the neutral line of the three- 
wire system. When the loads on the two sides of the system 
differ, a current flows in the neutral line and enters the armature 
through the reactance coil which offers only a small resistance 
to direct current. The currents in the sections M flow against 
the generated e.m.f. so that these sections are equivalent to the 
motor end of a balancer, while the currents in sections G flow in 
the direction of the generated e.m.f. and so these sections are 
equivalent to the generator end of a balancer. 

349. To transform from alternating to direct current, the 
motor-generator set may consist of either an induction or a 
synchronous motor, direct connected to a direct-current generator, 
The same transformation can be made in a single machine called 
the rotary converter. 

350. Rotary Converter. — Fig. 381 shows an alternator such as 
that on page 241 connected to a direct-current generator which has 
the same number of poles. If an alternating e.m.f. is applied to 
machine M, it will operate as a synchronous motor and direct 
current may then be obtained from machine G. The two ma- 
chines M and G may be combined to form a single machine as 
shown in Fig. 382, and, if an alternating voltage is applied to the 
slip rings ab, the machine will run as a synchronous motor 
while at the same time direct current may be obtained from the 
brushes at the commutator end. The resultant current in the 
armature will be the difference between the direct current I d 
drawn from the machine and the alternating current I a which the 
machine takes from the power mains. 

There is a fixed ratio between the voltages of the direct and 
alternating current sides of the machine. The voltage between 
the slip rings a and b is a maximum when x and y are in the 
neutral ^position and this must be the voltage between the 
brushes [c and d of the direct -current side therefore E d , the 
voltage of the direct-current end, is equal to the maximum volt- 



Art. 351] 



MOTOR-GENERATOR SETS 



319 



age at the alternating-current end or to ^2E a where E a is the 
effective voltage at the alternating-current end. 

It is impossible to change the voltage of the direct-current side 
of the machine without changing the alternating applied voltage. 
A change in the field excitation for example does not change the 
applied voltage, but it does change the phase angle of the current 
the converter takes from the line, see page 256, and a converter 
can be used for power factor correction in the same way as a 
synchronous motor, see page 257. 

If it is desired to raise the voltage of the direct-current side of 
the machine then the alternating applied voltage must be raised. 




Synchronous Motor 



Direct Current Generator 



Fig. 381. 




Rotary Converter 

Fig. 382. — Diagrammatic representation of a rotary converter. 



One method of doing this is to supply the rotary converter through 
a transformer which has taps on the secondary side by means of 
which the voltage may be raised or lowered. Another method is 
to insert a booster, generally on the alternating-current side of 
the machine. This booster consists of a small alternator, on the 
same shaft as the rotary converter, the voltage of which may be 
added to or subtracted from that of the alternating-current power 
mains. 

351. Motor-Generator Sets and Rotary Converters. — The 
principal points of difference between the rotary converter, the 



320 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xxxix 

induction motor -generator set and the synchronous motor-gen- 
erator set are as follows: 

Starting. — On the alternating-current side, the rotary con- 
verter operates as a synchronous motor and must therefore 
be brought tip to speed and synchronized, see page 258. The 
induction motor-generator set is self-starting. 

Parallel Operation. — A synchronous motor and a rotary con- 
verter are liable to hunt, see page 258, although this trouble is 
practically eliminated by the use of dampers. Induction motors 
do not hunt. 

Voltage Variation on the Direct-current Side. — In the case of 
the rotary, a booster or a boosting transformer is required to raise 
the voltage of the direct-current side whereas the voltage of the 
direct-current side of either type of motor-generator set can be 
controlled by the field excitation. 

Efficiency and Cost. — Being a single machine instead of two 
separate machines, the rotary converter is cheaper than either 
type of motor-generator set and its efficiency is the highest since 
it has only the constant losses (friction and iron loss) of one 
machine. 

Power Factor Control. — This is possible with the rotary con- 
verter and with the synchronous set but not with the induction 
motor-generator set. 

For small sizes, up to 100 kw., the induction motor-generator 
set is generally preferred because it is easily started and because 
the voltage of the direct-current end can easily be regulated. 
For large sizes, 500 kw. and over, a synchronous machine is 
preferred because it can be used to control the power factor of the 
system; if the voltage regulation is not of great importance, as in 
railway work, the rotary converter is used, but if a wide variation 
of voltage is required from the direct-current side, a motor- 
generator set is generally preferred. 

352. Polyphase Rotary Converter. — The only difference be- 
tween the single-phase and the polyphase machine is that the 
former is tapped at two points, as shown in Fig. 382, whereas the 
latter has to be tapped as if the machine was a polyphase alter- 
nator of the revolving armature type, see page 241. A three- 
phase rotary is shown in Fig. 383; the armature is tapped at three 
points, 120 electrical degrees apart. 

353. Split-pole Rotary Converter. — One method of changing 
the voltage of the direct-current side of a three-phase rotary con- 



Art. 354] 



MOTOR-GENERATOR SETS 



321 



verter would be to change the total flux which passes between 
ab, Fig. 384, without changing the flux which passes between ac. 
This is accomplished by splitting each pole as shown diagram- 
matically in Fig. 384 aud exciting each part separately. If then 
the excitation of the main pole M is unchanged, the flux threading 
ac will be unchanged, but the total flux threading ab may be 
increased or decreased by varying the excitation of R, and by 
this means the voltage of the direct-current side of the machine 
may be raised or lowered without any change being required in 
the alternating applied voltage. 




Fig. 383. — Three-phase rotary con- Fig. «384. — Split-pole rotary con- 
verter, verter. 

Another method of changing from alternating to direct current 
is described on page 357, the piece of apparatus by which the 
transformation is made is called the mercury vapor converter. 

354. Frequency Changers. — To change from one frequency 
to another, a motor-generator set must be used which consists of a 
synchronous motor direct connected to an alternating-current 
generator. If 60 cycles have to be obtained from a 25-cycle line, 
then the number of poles must be in the ratio of 60 to 25, and 
the smallest possible number of poles would be 10 for the motor 
and 24 for the alternator; this set would run at 300 r.p.m., so 
that such frequency changers cannot readily be made for small 
outputs. 



21 



CHAPTER XL 
ELECTRIC TRACTION 

An electric car with a seating capacity for fifty persons is gen- 
erally equipped with four motors each of 50 h. p. Such a powerful 
equipment is required to obtain the high speeds and high rates 
of acceleration used in electric traction. 

355. Tractive Effort. — The cycle of operations of an electric 
car with a reasonable distance between stops is shown in Fig. 385. 
During the time interval oa, energy is put into the motors and the 
car is accelerating; during the interval ab, the motor circuit is 




o a be x stop 

Fig. 385. — Cycle of operations of an electric car. 

open and the car is coasting; during the interval be the brakes 
are applied and the car gradually brought to rest. 

If W is the weight of the train or car in tons (2000 lb.) and a 
is the acceleration in miles per hour per sec. then the tractive 
effort in lb., required for acceleration 

Weight in lb. , ,. . ,, 

= X acceleration in it. per sec. per sec. 

9 
(W X 2000) a X 5280 
32.2 X 3600 
= 90 W X a 

An additional accelerating tractive effort of about 10 per cent, is 
required to supply the rotational kinetic energy of the gears, 
motor armatures and other rotating parts so that the tractive 
effort required for acceleration = 100 W X a = 100 lb. per ton 
for each mile per hour per sec. of acceleration; an acceleration 
of 1.5 miles per hour per sec. may be used without discomfort 
to the passengers. 

The tractive effort to overcome train friction (bearing, air and 

322 



Art. 356] ELECTRIC TRACTION 323 

rolling friction) is expressed in pounds per ton weight of train 
and is given by the empirical formula 

•50 „ , V 2 ( n 



f = ^w + °- 03 v + °- 0C2 A w{ 1+ 10 

where / is the tractive effort to overcome train friction in lb. per 
ton weight of train 
W is the weight of the train in tons (2000 lb.) 
V is the velocity of the train in miles per hour 
A is the cross section of the car above the axle in sq. ft. 

and is generally about 120 
n is the number of cars; the side friction of each additional 
car increases the air friction by 10 per cent. 

When there is a curve in the track, an additional tractive effort 
of 0.7 lb. per ton for each degree of curvature is required due to the 
increased rolling friction; the number of degrees of curvature is 
equal to 5730 divided by the radius of the curve in feet. 

When there is a grade an additional tractive effort of 20 lb. 
per ton is required for each 1 per cent, of grade. When the 
grade is 1 per cent, the car has to be lifted through 1/100 of the 
distance it travels and this is equivalent to lifting 1/100 of the 
weight, or 20 lb. per ton, through the whole distance travelled. 

The maximum tractive effort that, can be used without causing 
the driving wheels to slip is about 22 per cent, of the weight on 
these wheels, since maximum tractive effort = /x X weight on the 
driving wheels, where n, called the coefficient of adhesion, has the 
following values: 

for a clean dry rail = 0.28 

a greasy moist rail = O.lo or when rail sanded = 0.25 

a dry snow covered rail = 0.11 or when rail sanded = 0.15 

356. Speed Time Curve. — The characteristics of a traction 
oad can best be shown by the working out of an actual example. 

A 30-ton suburban car is equipped with four 50-h.p. motors having the 
characteristics shown in Fig. 386. The acceleration has to be 1.25 and the 
deceleration 1.5 miles per hour per sec; the road is level and without curves 
and the distance between stops 0.75 miles, the time of the stop 9 sec. and the 

1 Electric Traction by A. H. Armstrong, Standard Handbook for Electrical 
Engineers. 



324 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xl 



schedule speed 21 miles per hour. It is required to draw the speed time 
curve. 

f = Vw + 003F + 0002 x 120 x w( 1+ n fo ~) 

= 9 + 0.3 + 0.8 = 10. 1 lb. per ton at 10 miles per hour 

= 9 + 0.6 + 3.2 = 12.8 lb. per ton at 20 miles per hour 

= 9 + 0.9 + 7.2 = 17.1 lb. per ton at 30 miles per hour 

Now the effective tractive effort for an accleration of 

1.25 miles per hr. per sec. 

= 125 lb. per ton. 

The train friction during acceleration will be approximately 

= 11 lb. per ton. 

l herefore the total tractive effort required 

30 

1020 lb. per motor. 

Corresponding to this value on Fig. 386, 
the speed with 550 volts = 19 miles per hour 
and the current per motor = 100 amp. 



136 lb. per ton = 136 X -j- 



35 


1400 


k30 


31200 


o 

W 


3 


*B5 


^ 1000 


p. 


a 


W 




.2 20 


5 800 


H 




| 15 


2 600 










» 10 


H 400 



200 































X* 


V 












¥ 










s 


?/ 








<5 


/ 





















20 40 



Amperes 



100 120 



Fig. 386. — Characteristics of a 50-horse-power, 550-volt direct-current 
motor, the tractive effort and the speed being measured at the rim of the 
driving wheels of the car. 



The motor is so controlled that the current remains constant at 100 amp. 
per motor and the acceleration has a constant value of 1.25 miles per 
hour per sec. until the speed has become 19 miles per hour; the starting 
resistance has then all been cut out and normal voltage is applied to the 
motors. 

The time taken to attain this speed = vel./accel. = 19/1.25 = 15.2 sec. 

and the distance covered during this time = — ^~- X ocqa — 211ft. 

from these figures the points a, ai and a<i are plotted in Fig. 387. 

The motor, running on normal voltage, will now speed up, and the current 
will decrease. Let the current decrease to 60 amp. then: 
The corresponding tractive effort = 500 lb. per motor and the speed = 23 
miles per hour, Fig. 386. 



Art. 356] 



ELECTRIC TRACTION 



325 



The average tractive effort while the current is decreasing = — ~ — "~ " ' 

= 760 lb. per motor = 760 X 4/30 = 101 lb. per ton 
The train friction at 21 miles per hour = 13 lb. per ton 
The tractive effort available for acceleration = 88 lb. per ton 
The average acceleration = 88/100 = 0.88 mile per hour per sec. 

23 — 19 
The time taken for the speed to attain 23 miles per hour = ~~ TToo" - =4.6 sec. 

no _j_ I q P\9Sn 

The distance covered during this time = ~ X o^™ X 4.6 = 142 ft. 

The distance covered from the start = 211 + 142 = 353 ft. 

From these figures the points b, bi, and 6 2 , are plotted in Fig. 387; points 
c for a current of 40 amp. and d for 30 amp. are determined in a similar 
way. 

Now the schedule speed is 21 miles per hour. 

The distance covered is 0.75 mile. 



28 



100 3. 20 



S 16 



S. 60 S 12 



| 40> 
20 

















.^-- 


.-'- 


■n 














i/* 


*»' — 








1 




^- : 




■ 


^ 


'*C 1 












/' 








tti/ 


<hy 
















' V 






a A 










&', 








a\ 












1p 


r 










\ 


/ 


6 


\Cur 


rent 
C 
















\ 


/ 






"*<?2 




-_ 














\ 














d 


L 


"a 2 


bo 




















\ 



4800 



4200 



3600 



3000 a 



2400 



1800 



600 



10 20 30 40 50 60 70 80 90 100 110 120 
Seconds 

Fig. 387. — Speed-time curve of an electric car. 



The time per cycle = 0.75/21 = 0.0356 hour = 129 sec. of which 120 
sec. is the running time and 9 sec. the time of the stop. 

In order to bring the car to rest in 120 sec. from the start the brakes 
"must be applied at some point / such that the deceleration will be 1.5 
miles per hour per sec. 

If the distance curve be now completed it will be found that the car 
has travelled a distance of 4500 ft. In order that this distance be 0.75 
mile, power must be taken off the car at some point g and the car allowed 
to coast thereby reducing the average speed of the run. During coasting, 
the speed will fall off according to the curve gh. The slope of this line is 
determined as follows: the average velocity during coasting is 25 miles per 
hour and the corresponding retarding force due to train friction is 15 lb. 
per ton which will cause a deceleration of 0.15 mile per hour per sec. or a 
decrease in speed of 1.5 miles every 10 sec. 



326 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xl 



357. Energy Required by a Car. — The series parallel method 
of control, page 126, is used for railway motors so that between 
a and b, Fig. 388, the current taken from the line is twice the cur- 
rent per motor while between b and c four times the current per 
motor is taken from the line. 

The average current per cycle = 225 amp. 

225 X 550 , ^ n , 

= 123 kw. 



The average power per cycle 

The energy per cycle 

It is convenient to express the energy required by a car or 



1000 
123,000 X 44 

win = 1^00 watt-hr 



300 



a 200 



100 







~1 


























■ 












— 




i 


\ 


— 


— 




vSs'vW 






I; 5^s > ^ 


































b 


c 













20 30 10 
Seconds 



50 60 



1UU 

2 80 


































| 20 























2 4 6 8 10 

Distance between Stops in Miles 



Fig. 



388. — Current taken by an electric 
car during one cycle. 



Fig. 



389. — Energy consumed 
by an electric car. 



train in watt-hours per ton mile or in kilowatt-hours per car mile. 
These quantities are determined in the following way: 

The energy per cycle = 1500 watt-hours 

The distance travelled per cycle = 0.75 mile 

The weight of the car = 30 tons 

1500 



The watt-hours per ton mile = 



= 67 
0.75 



0.75X30 
The kilowatt-hours per car mile = ttj™ X tt^h = 2.0. 

These two quantities decrease as the distance between stops 
increases, as shown in Fig. 389, because then the energy required 
to accelerate the car becomes a smaller portion of the total energy 
per cycle. In the extreme case of city service, the coasting period 
is generally eliminated, the distance between stops being so short 
that only acceleration and braking are required. 

From curves such as those in Fig. 389, the power house capacity 



Art. 358] 



ELECTRIC TRACTION 



327 



may be determined once the number of cars and their schedule 
has been fixed. 

358. Characteristics Desired in Railway Motors. — The series 
motor is particularly suited for traction service, see page 103, 
because it develops the large starting torque required with the 
minimum current, see page 102. In addition, it takes light loads 
at a high speed and heavy loads at a slow speed, slows down on an 
up grade and speeds up on a down grade, it therefore maintains 
the load on the power house more uniform than if constant speed 
motors such as the direct- current shunt motor or the alternating- 
current induction motor were used. 

The alternating- current series motor, see page 310, has the same 
characteristics as the direct- current series motor but is more 



Main 

Poles 




Armature 
/nterpoles 



Fig. 390. — Direct-current street railway motor, 
expensive, has a lower efficiency and does not commutate so well. 
The method of control is simpler however, the low voltage re- 
quired for starting being obtained by means of an autotrans- 
former instead of by the use of resistance and the series-parallel 
connection. These motors can run on direct as well as on alter- 
nating current circuits. 

For one particular kind of service the polyphase induction 
motor is specially suited, even although it is a constant speed 
motor, and that is for mountain grade work where the grades are 
long and fairly uniform. When the car is on the down grade, the 
motors tend to speed up and run above synchronous speed, and, 
under these conditions they become induction generators and 
supply power back to the line. With a speed of 4 per cent, above 
synchronous speed, the machine will deliver its full-load rating 



328 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xl 




o 

43 



Art. 361] ELECTRIC TRACTION 329 

to the line, so that braking with brake shoes is not necessary and 
the danger of trouble on a long grade due to overheating of brake 
shoes is practically eliminated. 

359. Motor Construction. — To keep down the size of the motors 
they must run at a high speed, see page 101, so that gears are 
generally necessary. The standard construction for urban and 
interurban cars is shown in Fig. 390. 

Motors which are placed between the driving wheels, as are all 
street car motors, are restricted in size. This restriction is most 
keenly felt in the design of large horse-power slow-speed motors 
for electric locomotives for freight service. One method of 
providing increased space for the motors is shown in Fig. 391 
where two motors each of 2000 h.p. are mounted in the car 
and are connected to the driving wheels through side rods. 

360. Distribution to the Cars. — The three standard railway 
systems are shown diagrammatically in Figs. 392, 393 and 394. 1 

With the single-phase system the trolley voltage may be as 
high as 11,000 volts and in order to render high speed collection 
reliable with long spans the catenary construction is used, the 
trolley wire being carried from a steel messenger cable, the latter 
having a sag while the former lies parallel with the rails. 

Because of commutation troubles the standard direct- current 
voltage is not higher than 600 volts although there are lines in 
operation at 1200 volts and a few experimental lines at 2400 volts. 
Large currents are therefore necessary for electric trains and to 
carry this current a third rail insulated from the ground is used 
instead of the trolley wire if the trains run on a private right of 
way. For city service the familiar trolley is used and the 
trolley system is supplied by feeders from substations. 

The return circuit is through the rails in each case. This cir- 
cuit is made as highly conducting as possible by bridging all the 
rail joints with flexible pieces of copper called bonds which are 
electrically welded to the side of the rail. 

361. Alternating- and Direct-current Traction. — The single- 
phase alternating current system has the advantage that voltages 
as high as 11,000 volts may be used on the trolley so that the 
current in the trolley wire is small and power may be transmitted 
for long distances before the voltage drop becomes too large, 
the substations therefore are few in number and contain merely 

1 Taken from a paper by George Westinghouse, in the Trans, of Ameri- 
can Soc. of Mech. Eng., July 1910. 

22 



330 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xl 






Art. 361] 



ELECTRIC TRACTION 



331 




332 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xl 

the necessary step-down transformers. For the direct- current 
system, on the other hand, at 600 volts, substations must be closer 
together to maintain the trolle}^ voltage and these substations 
contain rotary converters as well as step- down transformers. 

The principal disadvantages of the alternating-current system 
are that the motors are heavier than direct- current motors of 
the same horse-power, while the alternating magnetic flux link- 
ing the trolley causes interference with adjoining telephone 
systems. 

362. Motor Car Trains. — A number of cars each with its own 
motors and controller may be joined together to form a train 
which, by means of the multiple unit system of control, see page 
133, can be controlled by a single operator at the head of the train. 
This system of operation is very flexible because cars can be 
added to suit the traffic while trains can readily be split up at 
junctions or single cars can be attached or dropped as required 
without the necessity of stopping the whole train. 

The acceleration can be rapid because all the weight is on 
the drivers and a large draw bar pull can be obtained. Heavier 
trains can also be run at higher speeds than by locomotives, while 
the wear and tear on the track is reduced because the weight is so 
well distributed. Such trains can be run in either direction 
without having to be turned end for end. 

363. Electric Locomotives. — When the cars of a steam road 
have to be hauled into the city, through tunnels, or on mountain 
grades, electric locomotives are often used. Their capacity is not 
limited by grate area and boiler capacity so that they can develop 
very large torques for starting and accelerating and can generally 
give 50 per cent, more draw bar pull than a steam locomotive 
of the same weight. The torque also is uniform, such locomo- 
tives can therefore give high acceleration and high schedule 
speeds. 

ELECTRIC HOISTING 

364. Crane and Hoist Motors. — In the case of cranes, where 
the load is visible at all times and where the starting and accelerat- 
ing of the load is a large part of the total hoisting cycle, the 
direct current series motor is used because of its good starting 
characteristics. Where only alternating current is available, 
the wound rotor polyphase induction motor is generally used, but 
it is not such a satisfactory machine, see page 298. 



Wheel 



Art. 365] ELECTRIC TRACTION 333 

Motors for large cranes, such as those used in rolling mills, 
take large currents and are generally controlled by magnetic 
switch controllers. 

When the load has to be raised a considerable distance, as in 
the case of mine hoisting, the load is accelerated for only a small 
portion of the total cycle and is run thereafter at a constant speed 
not greater than that fixed by law. For such service the direct- 
current compound motor is suitable because its speed cannot 
exceed a safe maximum value. Where only alternating current 
is available, the polyphase wound rotor induction motor is used. 

365. Braking. — -The brakes used on cranes and hoisting 
machines must be so constructed that they will set if there is any 
interruption in the current supply. Such a brake is shown in 
Fig. 395. When power is switched on to start the hoisting motor, 
the solenoid lifts the lever L 
and releases the brake, but as 
soon as current ceases to flow, 
the weight W sets the brake. 

When a light load is being 
lowered, power must be ap- 
plied to drive it down. When FlG 39 5._ Brake for hoists and cranes, 
the load is heavy, it may be 

allowed to overhaul the motor, the speed being kept within rea- 
sonable limits by means of the brake. This causes jerky opera- 
tion, and excessive wear of the brake, and, for coal and ore un- 
loaders and other such hoists doing fast hoisting service, mechan- 
ical braking is not veiy satisfactory and dynamic braking is now 
used. 

The hoisting motors for such hoists are direct-current compound 
wound and are connected as in Fig. 396 during the hoisting period. 
When being lowered, the empty bucket is allowed to overhaul the 
motor, which is connected as shown in Fig. 397, the shunt coils 
being separately excited while the armature is short circuited 
through an adjustable resistance R. The machine then acts as a 
generator driven by the descending load and sends a current I 
through the resistance R. As the machine speeds up, the current 
I increases until the retarding torque due to the current I becomes 
equal to the effective torque driving the armature. 

To raise the lowering speed, the resistance R must be increased. 
This reduces the current / and the braking torque temporarily, 
and the speed increases until the greater e.m.f. generated in the 





" 


Solenoid 


Weight 
[ ( W / 


L 1 _aCs 


\ 



334 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xl 

armature is again able to send the necessary current I through 
the higher resistance. By varying the resistance R, the lowering 
speed may be varied from almost zero to any desired value. To 
stop the load, a mechanical brake must be applied, since dynamic 
braking can only take place while the armature is moving. 




w 



w 



Fig. 396. — Hoisting period 
Figs. 396 and 397. 



Fig. 397. — Lowering period, with 
dynamic braking. 
Connections of a compound-wound direct-current 
hoist motor. 



Dynamic braking is also applied to crane motors. During 
hoisting, the motor is connected as shown in Fig. 398 On the first 
step of the controller about two-thirds of full-load current flows 
through the circuit, and this is sufficient to release the brake and 
start the average load. 




Fig. 398.— Hoisting. Fig. 399.— Lowering. Fig. 400.— Dynamic 

braking. 
Figs. 398 to 400. — Connection of a series-wound direct-current crane motor. 



When lowering, the machine is changed over to a shunt machine 
as shown in Fig. 399 and a resistance R is inserted to limit the 
flow of current in the field coil circuit to two-thirds of full-load 
current, which is sufficient to release the brake. The direction of 



Art. 366] 



ELECTRIC TRACTION 



335 



the field current is the same as during hoisting, but the armature 
terminals are reversed so that the armature current is reversed 
and the motor drives the load down. If the load is heavy enough 
to overhaul the motor, then the motor speeds up and its back 
e.m.f. increases, until finally the machine runs as a generator and 
acts as a brake. The current I a is now reversed and the machine 
supplies its own exciting current, so that it may be disconnected 
from the line as shown in Fig. 400 and the braking speed adjusted 
by means of the rheostat Rb. 

366. Flywheel Motor Generator Sets for Mine Hoisting. — The 
load on the motor of a hoist is extremely variable, as shown by 
curve A, Fig. 401, and, when the motor has an output of 500 
h.p. or more as in the case of motors for mine hoists, such 
a load is not a very desirable one for a power company, and a 



320 


~-T- 






-h-i+ 






300 


:aj- 


::: |^______^. 






----- JE— — : 


:::::j::: 




-4L 










i it 


f 




it 










it t 


r 




jt 




it i 






i ttt 


± h 




t 




£ i 






i tit t 






± 




r i 




jrr 


1 






± 




jt _ ±] 


40 


it 


itt 






h Ji 






fi-LL 


XX 


it its. 


_ >;_ 




"IC*... 






v it *-*i" 


fl 1 *■- 


it\. _ ■ 










i \\\ -jjji \i 
























£n 






















~X- \ ti 


I it 






it 










i it 






it 








± ~L__i± 


r___rt 




lit _L-L__ 


D 



OOO OOOOOOO 



o o o o o 



o o o o 



©OOOOOOO 



Time in Seconds 
Curve A Current taken by hoist motor. 
Curve B Current taken by induction motor. 



Fig. 401. 



-Characteristics of a hoisting load when a fly-wheel motor- 
generator set is used. 



cheaper rate for power can generally be obtained if the load is 
more uniform. 

To obtain this result and at the same time to reduce the large 
losses in the starting rheostat, the Ward Leonard system, see 
page 110, is used. The high speed motor generator set consists 
of a wound rotor induction motor which takes power from the 
transmission line, and a direct-current generator which supplies 
power to the hoisting motor, while a heavy flywheel is mounted 
on the same shaft as shown in Figs. 402 and 403. 

The excitation of the hoisting motor is kept constant by means 
of the exciter E, while the speed of the motor is controlled by the 
resistance r in the generator field circuit, by means of which the 
voltage E t applied to the motor terminals may be varied. 

As the load on the hoist motor increases, the current in the 
induction motor leads tends to increase in the same ratio, but a 
slight increase in the current J causes the plunger p of the sole- 



336 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xli 



noid S to raise the plates of the water rheostat W thereby increas- 
ing the resistance in the rotor circuit, so that the rotor current 
and torque are maintained constant. This torque is not sufficient 



Induction Motor D.CGenerator 



Exciter 




Fig. 402. — Flywheel motor-generator set for the operation of mine hoists. 

for the load, so that the speed drops and thereby causes the fly - 
wheel to give up energy. 



J d 



Main Switch 

with Overload 

Eelease 



Induction Motor 




Hoisting 
Motor 



T 

Safety 
Stop 



TP3§ S 



Flywheel Motor-Generator Set 

Fig. 403. — Connections for a hoist motor supplied by a fly-wheel motor 

generator set. 

If the load on the hoist motor now decreases, the current I 
tends to decrease, but a slight decrease in this current causes the 
pull of solenoid S to decrease and the plates of the water rheostat 



Art. 367] ELECTRIC TRACTION 337 

to drop, thereby decreasing the resistance in the rotor circuit, so 
that the rotor current and the rotor torque are maintained. 
This torque is greater than necessary for the load and so acceler- 
ates the flywheel. 

By means of such a slip regulator, the power taken from the 
line is maintained approximately constant, as shown in curve B, 
Fig. 401. 

367. Safety Devices. — The safety brake is of the type shown 
in Fig. 395 and will set and hold the load as soon as current ceases 
to flow in the solenoid B. 

If for example the field circuit of the hoisting motor were to 
open, the current in the solenoid B would be interrupted and the 
brake would set. 

If the cage overtravels, it closes the switch T in the hatchway 
and this shunts the current from the solenoid B and allows the 
brake to set. The resulting overload then opens the circuit 
breaker C. 

The solenoid B is also shunted when, due to an excessive over- 
load, the circuit breaker C opens and closes the contacts ab. A 
similar pair of contacts cd ensure that the brake shall be set when 
the main switch is open. 



CHAPTER XLI 
TRANSMISSION AND DISTRIBUTION 

368. Direct-current Stations. — When direct current is used, 
the voltage can be transformed up and down only by means of 
motor generator sets and, since these are expensive and require 
supervision, they are seldom used, so that the connected load of 
motors and lamps must operate at the power house voltage. 

The 110-volt lamp is practically standard because it has a 
stronger filament than have lamps of higher voltage. The 
use of such a low voltage necessitates the use of conductors of 
large cross section to carry the current, and higher voltages are 
desirable. 

If 50 kw. has to be transmitted a distance of 100 yd. with a drop in vol- 
tage not exceeding 2.5 per cent., find the necessary cross section of the con- 
ductor if the voltage is 110 volts and also if it is 220 volts. The resistance 
of copper is taken as 11 ohms per cir. mil foot. 



At 110 volts 



At 220 volts 



Current in the line 



50 X 1000 



110 
Voltage drop in the! = 2.5% of 110 
volts 
2.75 



= 455 amp. 
2.75 



50 X 1000 



line. 
Resistance of the line. 



Resistance of 200 yd. 

of wire. 
Cross section of wire in 

cir. mils. 



0.006 ohms 

= 0.006 



455 

11 X200 X3 



220 

= 2.5% of 220 
volts 

5.5 



227 amp. 
5.5 



, 7 = 0.024 ohms 
11 X200 X3 



cir. mils 
1,100,000 



cir. mils 
= 275,000 



= 0.024 



The cross section of the wire is inversely proportional to the 
square of the voltage for the same loss in the line. 

If the three- wire system of distribution is used then 110 volt 
lamps may be operated from 220 volt mains as shown in Fig. 376, 
page 316. The neutral wire may be small in cross section if the 
load is nearly balanced under all conditions of loading. 

Where there is only a small lighting load, or where the lights 

338 



Art. 369] 



TRANSMISSION AND DISTRIBUTION 



339 



are supplied from special mains, then a voltage of 550 may be 
used for the motors. Voltages greater than 550 are dangerous; 
even 110 volts may prove fatal to a person who happens to make 
unusually good contact with the mains. 

369. Alternating-current Stations. — Where power has to be 
transmitted a long distance or has to be distributed over a wide 
area, the alternating- current system is preferred because the 
voltage can be transformed up and down by means of transform- 
ers. These require no supervision and may be installed in the 
open, on poles or in manholes. 

A typical high- voltage transmission system is shown in Fig. 
404. The power station contains the generators and step- up 




Generator 
2200 Volt 



<j>| Step up 
Transformers 



/ Oil Switch 




13,200 Volts 
to Substations 



Power House 



Terminal Station 



Fig. 404. — Diagrammatic representation of a high voltage transmission 

system. 



transformers while the step- down transformers are placed in a 
terminal station. The load is carried by several units operating 
in parallel so that one machine may be shut down for repair 
without affecting the total load. 

From the terminal station, power is transmitted to a number of 
substations which are conveniently located with respect to the 
load. In the case of a transformer substation, the equipment 
consists of the necessary step-down transformers to reduce the 
voltage to 2200 volts at which it is supplied to the feeders. Each 
feeder is generally controlled by a feeder regulator, see page 281, 
by means of which its voltage may be adjusted. These feeders 



340 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xli 

supply transformers which are placed on poles, or underground 
in manholes, and step the voltage down to 110 volts for the 
consumer. 

When it is necessary to transform to direct current as for exam- 
ple to supply power to a large machine shop, the substation equip- 
ment is as shown in Fig. 405. Power enters the station over the 
three bare wires a, b, and c which are carried through porcelain 



•^y a U Choke/Coils 

Bushine , „, *. /„ 




m n p 

Fig. 405. — Connections of an A.-C. to D.-C. motor-generator sub-station. 



wall bushings and are connected to the high-tension bus bars m, 
n and p. Power to operate the synchronous motor is taken from 
these bus bars through a delta-connected bank of transformers by 
which the voltage is reduced to 2200 volts, while half voltage 
taps are also supplied so that the self-starting motor may be 
started at half voltage; the double-throw switch for this purpose 
is not shown. 



Art. 371] TRANSMISSION AND DISTRIBUTION 341 

370. The voltages used in practice are : 

Direct Current: 

110 volts for lighting, generally obtained from a 220-volt three- 
wire system. 

110, 220 and 550 volts for motors. 

600 volts for street railway systems. 

1200 volts for interurban systems. 

2400 volts for trunk-line electrification. 
Alternating Current: 

110 volts single phase for lighting and for small motors. 

110, 220, 440 and 550 volts for polyphase motors up to 50 h.p. 

440, 550 and 2200 volts for polyphase motors greater than 
50 h.p. 

13,200 volts is the highest voltage generated by alternators; 
low -voltage alternators with step-up transformers are more 
reliable. 

100 volts per mile of line with a maximum of 110,000 volts for 
power transmission. 

The tendency is to use a frequency of 60 cycles for power and 
lighting work as it gives a better choice of speeds for induction 
motors than does 25 cycles, see page 297. For single-phase rail- 
way work, however, 25 cycles are necessary because of the diffi- 
culty in constructing motors that will commutate satisfactorily 
at a higher frequency. In the case of cement mills and other 
such places where most of the induction motors are slow-speed 
machines, 25 cycles may be used with advantage. 

371. Comparison between single-phase and three-phase 
transmission. 

10,000 kw. at 80 per cent, power factor has to be delivered at the end of a 
single-phase 25-mile line at 50,000 volts and 60 cycles. The size of wire is 
No. 000 and the wires are spaced 72 in. apart. Find the voltage at the 
generating station and also the power loss in the line. 

the resistance of this wire = 0.326 ohms per mile, page 216. 

the reactance at 60 cycles = . 742 ohms per mile, page 216. 

+ , .... .. 10,000 X 1000 OK _ 

the current m the line = fl ~ 50 00n~ = ^ amp. 

the resistance drop in 50 miles of wire == IR 

= 250 X 0.326 X 50 

= 4075 volts 
the reactance drop in 50 miles of wire = IX 

= 250 X 0.742 X 50 

= 9250 volts, 
the voltage of the generating station = E u , Fig. 406. 



342 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xli 



the power loss in the line 



= V C&cos a. + IRY + C&sin a. + 7X) 2 

= V (40,000 + 4075) 2 +(30,000 + 9250) 2 

= 59,000 volts 

= PR 

= 250 2 X 0.326 X 50 

= 1020 kw. 

= 10.2 per cent, of the output 

15,000 kw. at 80 per cent, power factor has to be delivered at the end of 
a three-phase 25-mile line at 50,000 volts and 60 cycles. The size of wire 




>-/ 




Neutral Wire Carries No Current 



Fig. 406. 



Fig. 407. 



is No. 000 and the wires are spaced 72 in. apart. Find the voltage in the 
generating station and also the power loss in the line. 



the resistance of the wire = 0.326 ohms per mile, 

the reactance at 60 cycles = 0.742 ohms per mile. 

15,000 X 1000 



the current in the line 



0.8 X 1.73 X 50,000 



= 217 amp. 



The problem is best solved by considering each line separately as shown in 
Fig. 407 then 

E t , the line voltage to neutral = 50,000/1.73 = 29,000 volts 
the resistance drop in 25 miles of wire = IR 

= 217 X 0.326 X 25 

= 1770 volts 
the reactance drop in 25 miles of wire = IX 

= 217 X 0.742 X 25 

= 4020 volts 
the line voltage to neutral at the generating station 

=- V (29,000 X 0.8 + 1770) 2 + (29,000 

X 0.6 + 4020) 2 
= 32,950 volts 
= 32,950 X 1.73 
= 57,000 volts 
= 3 X 217 2 X 0.326 X 25 
= 1150 kw. 
= 7.6 per cent, of the output. 



the voltage between lines 
the power loss in the line 



Art. 372] TRANSMISSION AND DISTRIBUTION 



343 



372. Lightning arresters are used to protect electrical equip- 
ment from lightning discharges and abnormally high voltages 
of all kinds. 

The current due to a lightning discharge has a high frequency 
and will not pass readily through a reac- 
tance, so that, if, as in Fig. 408, a resis- 
tance path R is provided to ground with 
an air gap g long enough to prevent the 
flow of current under normal conditions, 
and a reactance or choke coil C is placed 
between the line and the equipment to be 
protected, then a lightning discharge will 
be held up by the choke coil and will 
jump across the air gap to ground. 

Once an arc is started across the gap, 
however, the line current will follow 
through the path abed, and provision 
arrester to prevent this current from 




1* 



I 



Fi< 



. 408. — Connection 
lightning arresters. 



of 



must be made in the 
passing. The current 
may be limited by inserting a resistance R in series with the gap, 
while, if the electrodes of the air gap are made of non-arcing metal, 



! 

1 

a: 


_ f 


E 

Sri v: 
1 


8 

ft 

o 
-J 



Blow-out 
Coil y 



Fig. 409.— 3000 volt 
multigap arrester for sta- 
tion installation. 



Fig. 



410. — Lightning arrester for direct-current 
circuits. 



that is metal such as zinc which has a low boiling point, then, 
on an alternating- current line, the arc will not be maintained but 
will stop as the alternating current passes through zero. A single 
gap of non-arcing metal will protect a 300-volt line, for higher 
voltages, several gaps are placed in series as in the 3000-volt 



344 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xli 



arrester shown in Fig. 409 which is really equivalent to three 
arresters one with 3 gaps and a high resistance in series, an- 
other with 6 gaps and a lower resistance in series, and the third 
with 15 gaps in series. 

It is not so easy to rupture a direct current and, even when non- 
arcing electrodes are used, it is necessary also to supply a blow-out 
coil connected as shown in Fig. 410, which is excited when a power 
current flows through the arrester. 



To Horn Gap '\jL 




Fig.^ 411. —Aluminium arrester. 

For the protection of long-distance high- voltage transmission 
lines the aluminium arrester is generally used. An aluminium 
cell consisting of two aluminium plates on which a film of hydrox- 
ide has been formed, when immersed in a suitable electrolyte, 
will allow only a very small current to flow, until the voltage 
reaches a critical value. At a higher voltage, the current that 
can flow is very large but the high resistance is reestablished as 



Art. 373] 



TRANSMISSION AND DISTRIBUTION 



345 



soon as the voltage is reduced below the critical value. Such 
a cell can therefore act as a safety valve and aluminium arresters 
are made up in the form shown in Fig. 411, about 300 volts per 
pair of plates being allowed. 

Even with 300 volts between plates, a small current flows, to 
prevent which, the arrester is connected to the line through a 
horn gap as shown in Fig. 404. When the arrester is disconnected 
from the line, however, the hydroxide film dissolves, so that the 
arrester must be charged daily by being connected to the line 
by the closing of the horn gap for a few seconds. 




Fig. 412. — Hand-operated, triple-pole oil switch. 



373. Switches. — Oil switches such as that shown in Fig. 412 
are used to rupture the current in high-voltage lines. These 
switches are generally kept at a distance from the operator and 
are opened and closed through a system of levers, or may be of the 
remote control type operated by means of a solenoid or by a small 
motor. In all cases the switch is closed against the tension of a 
spring and is held closed by means of a latch. This latch may 
be released by an overload relay, so as to allow the switch to open 
when the current becomes excessive. 

To localize trouble, important switches are generally mounted 
as shown in Fig. 413 with each pole in a separate brick or concrete 
compartment. 

Disconnecting switches such as that shown in Fig. 405 are not 

23 



346 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xli 

intended to open while current is flowing but are used to discon- 
nect apparatus once the circuit has been opened by an oil switch. 
Such disconnecting switches are opened and closed by a long stick 
with a hook attached to the end. 

374. Overhead Line Construction. — For voltages up to 50,000, 
wooden poles with pin insulators are used to support the line. 
For higher voltages, pin insulators become very large and the 




Fig. 413. — Motor-operated, triple-pole oil switch. 



stresses on the pin become excessive, so that the suspension type 
of insulator has to be used and these are generally suspended from 
steel towers as shown in Fig. 415. 

To protect the line from lightning, it is usual to run a steel wire 
parallel to the power wires, and to ground this wire at every tower. 
Lightning will generally strike this ground wire and pass to the 



Art. 375] 



TRANSMISSION AND DISTRIBUTION 



347 



ground without doing injury, rather than strike the power wires 
and then pass to ground through the insulators. 

375. Underground Construction. — To carry current under- 
ground, stranded copper cable is used. The copper is insulated 
with paper which is then impregnated with a compound such as 



Section A-D 




Section C-D 

-Ground Wires 






Fig. 414. — Wooden pole with pin in- 
sulators, and ground wire at top. 



Fig. 415. — Steel tower with suspen- 
sion insulators. 



resin oil after which the cable is sheathed with lead which keeps 
out moisture and at the same time protects the cable against 
mechanical injury. The cable has to be flexible enough to bend 
round corners because it has to be drawn into tile ducts through 
manholes such as that shown in Fig. 416, which manholes are 
restricted in size. 



348 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xli 



The necessary cross section of copper is generally fixed by the 
permissible voltage drop in the case of low- voltage cables, but is 
always fixed by heating in high-voltage cables. A current 
density of 1000 amp. per sq. in. of copper section can 
seldom be exceeded, and this requires 8.7 volts per 500 ft., 
which is 2.5 per cent, of 350 volts, so that, if the voltage drop is 
limited to 2.5 per cent, and the transmission distance is 1000 ft., 
then, for voltages less than 350 volts, the current density must be 
less than 1000 amp. per sq. in. while for voltages greater than 
350 volts, the drop in the cable will be less than 2.5 per cent. 



F6 



Street Level 



Fig. 416.— Manhole. 

376. Switchboards. — For convenience of manipulation, all the 
apparatus for controlling the machines and circuits in a power 
station, as well as all the measuring instruments, are assembled 
as compactly as possible on a switchboard. 

The method of designing a switchboard is first of all to lay 
out the complete diagram of connections, then design the front 
of the board placing the different pieces of apparatus in the most 
convenient positions, after which the connections on the back of 
the board can be laid out and any rearrangement necessary can 
then be made. 

Fig. 417 shows the diagram of connections for a single shunt 
generator which supplies four feeders. 

Fig. 418 shows the front of the board. 

Fig. 419 shows the connections as they would appear if the 
slate front of the board was removed. This diagram is lettered 
similarly to Fig. 417. 

In larger stations it is usual to provide a separate panel on the 
switchboard for each machine and also for each feeder. 

Fig. 420 shows a three panel switchboard for a three phase alter- 
nator, a three phase feeder and an exciter. 



Art. 376] 



TRANSMISSION AND DISTRIBUTION 



349 



The exciter panel is equipped with: 

1 ammeter A\. 

1 hand wheel for the exciter rheostat Ri. 

1 switch S, with a fuse which is on the back of the panel. 

2 switches Si and $ 2 for the station lighting and other auxiliary 
circuits. 

d 




Fig. 418. Fig. 419. 

Figs. 417 to 419. — Switchboard for a direct-current shunt generator, 

and four feeder circuits. 



The exciter voltage is indicated by the voltmeter V\ on the 
swinging bracket. As the station grows in size, a second exciter 
will have to be added to operate in parallel with the first but, by 
means of a plug P, the voltmeter Vi may be made to indicate 
the voltage of each machine. 



350 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap. xlI 

The generator panel is equipped with : 
1 three phase indicating wattmeter. 
1 ammeter. 
1 voltmeter. 
. 1 three phase watt-hour meter, (called in practice a record- 
ing wattmeter.) 

1 handwheel for the alternator field rheostat R 2 . 

1 field switch S 3 . 

1 triple pole single throw (T. P. S. T.) oil switch. 

1 current transformer C. 

2 potential transformers V. 



3yuchroriett$f>e 
i Syactimry'zim? Lumps 

Ammeter A / f Mmtttr 



Ammeier 




■ $mkh 
•iih Overload &eky 



3We€hl Smith 

/feted 



To Cm-rent " |i 
Coils of Instruments 



f£U 



f 






.!ir 



3 Pole 
Oil Switch 



3 Phase 
Alternator 

Lightning |~ 
Arrester m 



3 Pole 
' Oil Switch 



Ammeter 
Ammeter 
Ammeter 
Overload Relay 
for Oil Switch 



Outgoing 
Lines 



Switchboard Connection diagram 

Fig. 420. — Switchboard with an exciter panel, a three-phase alternator and 
a three-phase feeder panel. 



As the station grows in size, additional alternators have to be 
added and they must operate in parallel so that a synchroscope 
must be provided. This is placed on the swinging bracket and 
the necessary connections are made by plugs on the generator 
panels. For a single alternator a synchroscope is not required. 

The generator oil switch has no overload release; protection 
against overloads is provided for by the use of automatic switches 
on the feeder circuits. 

The feeder panel is equipped with : 

3 ammeters. 

1 T.P.S.T. oil switch with overload release, 

3 current transformers. 



Art. 377] 



TRANSMISSION AND DISTRIBUTION 



351 



377. Instrument Transformers. — The instruments in circuits 
with a voltage of 2300 volts or greater are not connected directly 
in the circuit but are connected through transformers as shown 
diagrammatically in Fig. 421. 

T is a potential transformer and is built exactly like a standard 
lighting transformer but on a 
smaller scale. The voltmeter V 
measures the secondary voltage 
but is calibrated in terms of the 
voltage of the primary side. 

The series transformer has the 
primary side connected directly in 
the line and the secondary short 
circuited by an ammeter or by the 
current coils of a wattmeter. 

Since the secondary ampere turns of a transformer are always 
equal in number to the primary ampere turns therefore n 2 h = 

niIiorI 2 = —Ii so that the current measured by the instruments 
is proportional to the current in the line. 




Fig. 421. — Connections of in- 
strument transformers. 



CHAPTER XLII 
ELECTRIC LIGHTING 

A hot body gives off radiant energy in the form of heat, light 
and chemical energy and, to maintain the temperature of the 
body, energy must be given to the body at the same rate as it is 
dissipated by the body. An illuminant should have as much of 
the total radiation as possible in the form of light; power is 
required to maintain the other radiation but no light is obtained 
from it. 

As the temperature of a body is raised, the color of its light 
changes from red to white, while its light efficiency increases 
rapidly, so that high temperature is a necessary condition for an 
illuminant of the incandescent type. 

378. The carbon incandescent lamp consists of a filament of 
carbon enclosed in a glass globe from which the air has been ex- 
hausted. When the temperature of such a filament reaches about 
1600° C, the rate of evaporation of the carbon becomes excessive 
and the life of the filament becomes short. The life of such a 
lamp is the number of hours the lamp will burn before its candle 
power drops to 80 per cent, of the original value; the decrease in 
candle power is due largely to evaporation of carbon from the 
filament, this carbon deposits on the inside of the globe and 
blackens it. 

If the voltage applied to a carbon lamp increases, the current 
increases, and so also does the temperature of the filament and 
the efficiency of the lamp, but the life is reduced. A standard 
16 candle-power, 110 volt lamp takes 50 watts, or 3.1 watts per 
candle-power, with a life of about 500 hours; if the voltage is 
increased 2 per cent., the efficiency is increased about 7 per cent, 
and the life is reduced about 40 per cent. 

379. The Tungsten Lamp has a filament of metallic tungsten. 
The temperature of such a filament can be maintained at about 
2000° C, which is higher than the operating temperature of a car- 
bon filament, so that the tungsten lamp is the more efficient, taking 
only 1.2 watts per candle-power, and gives the whiter light. The 

352 



Art. 380] 



ELECTRIC LIGHTING 



353 



tungsten lamp is the more fragile of the two and the life of the 
lamp is not limited by a decrease in candle-power, but by the 
wear of the filament, which causes it to break after about 1000 
hours of service. 

A low- voltage lamp is more robust than a high- voltage lamp 
of the same candle-power, because the filament is shorter and of 
larger cross section, since it has to carry a larger current with a 
smaller voltage drop. 

One important difference between tungsten and carbon is that, 
while the temperature coefficient of resistance of the former is 
positive, that of the latter is negative. Because of this, the 
tungsten lamp is less sensitive than the carbon lamp to voltage 
fluctuations. If for example the voltage is increased by k per 
cent., the corresponding increase of current in the tungsten lamp 
will be less than k per cent", because the resistance of the filament 
increases, while the increase of current in the carbon lamp will be 
greater than k per cent, because the resistance decreases. The 
effect of an increase in voltage is shown in the following table : 



Voltage 


Candle-power 


Watts per candle- 
power 


Life 


Normal or 100% 
102% 

98% 


100% 
111% 
107% 

90% 

93.3% 


100% 

93% 

96.3% 
106% 
103.7% 


100% 

60% carbon 
76% tungsten 
147% carbon 
125% tungsten 



Because of the positive temperature coefficient of resistance, 
the tungsten lamp has a much lower resistance when cold than 
when hot, so that, when the lamp is switched on, the initial current 
is several times as large as the normal operating current. This 
result, called overshooting, reduces the life of a lamp if it is 
switched off and on frequently; for sign lighting, low- voltage 
lamps are used since they have a stouter filament than standard 
110- volt lamps. 

380. Gas-filled Tungsten Lamp. — If tungsten is heated in an 
atmosphere of nitrogen instead of in a vacuum, the temperature 
at which evaporation becomes excessive is higher in the former 
case than in the latter. Such nitrogen- filled lamps therefore have 
a high efficiency and, in large sizes, take only 0.5 watts per candle- 
power. Since the globe contains gas, convection currents are set 



354 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xlii 

up when the lamp is lit, and the globe has the comparatively high 
temperature of about 200° C. 

To obtain high efficiency from this lamp, it is found that the 
filament must be large in cross section; a filament to carry 20 
amp. at 10 volts gives 2 candle-power per watt while a thinner 
filament to carry 5 amp. at 40 volts gives only 1.4 candle-power 
per watt. The lamp at its best efficiency cannot be made at 
present for less than about 350 candle-power, in which size it is 
suitable for the lighting of large areas at present lit by arc lamps 
of lower efficiency. 

381. The Unit of Light. — The light giving power of a lamp is 
expressed in candle power. This, however, has little meaning 





Fig. 422. — Light distribution in a 
vertical plane, from a 32 candle-power 
tungsten lamp. 



Fig. 423.— One type of 
arc-lamp mechanism. 



unless the direction of the light is specified, and curves such as 
that in Fig. 422 are used for this purpose. This curve shows the 
distribution of light about the vertical plane of a tungsten lamp, 
and is obtained by measuring the candle power in different direc 
tions and then plotting along each radius a length proportional to 
the candle power in that direction. 

Incandescent lamps are generally rated in mean horizontal 
candle power. Thus the lamp on which the curve in Fig. 422 
was taken would be rated at 32 candle power although the average 
candle power in all directions, called the mean spherical candle 
power (m.s.c.p.) is only about 24 m.s.c.p. 

382, Arc Lamps. — If two sticks of carbon connected in an 
electric circuit as shown in Fig. 423 are brought into contact, a 



Art. 384] ELECTRIC LIGHTING 355 

current will flow through the circuit and, since the contact be- 
tween the carbons is poor and the resistance of the contact is 
therefore high, the carbons at the contact begin to glow, while a 
small quantity of carbon vapor passes between them. 

If the carbon contacts are now separated by about a quarter of 
an inch, it will be found that the current still flows, because the 
space between the contacts is filled with carbon vapor which is 
conducting. The arc so formed is a powerful source of light. 

An arc lamp consists of two sticks of carbon, with a mechanism 
which, when the voltage is applied, brings the carbons into con- 
tact and then separates them, and which also feeds the carbons 
together as they are consumed. 

One of the many types of mechanism for this purpose is shown 
diagrammatically in Fig. 423. The upward pull of the shunt 
coil S tends to bring the carbons together, and this pull increases 
with the voltage E; the upward pull of the series coil L increases 
with the current i" and tends to separate the carbons. When the 
main switch is closed, a large current I passes through the coil L 
while the voltage E is comparatively small, so that the pull of L 
is greater than that of S and the carbons are separated. As 
they separate, E increases and I decreases and, when the arc 
has reached the proper length, then E and I have their normal 
values and the pulls balance. 

383. The direct-current open arc takes the form shown in 
Fig. 425. The temperature of the positive tip is about 3700° C, 
the temperatures of the arc stream and of the negative tip are 
much lower. Of the total light from such an arc, 85 per cent, 
comes from the crater, 10 per cent, from the positive tip and 
5 per cent, from the arc stream. The distribution of light from 
such an arc is shown by the polar curve in Fig. 424; directly below 
the arc the illumination is practically zero due to the shadow cast 
by the lower carbon. 

Because of the high temperature, the light is white and the 
efficiency is high, but the life of the carbons is only about 10 hours. 

384. Direct-current Enclosed Arc. — To cut down the trimming 
expense, the arc is enclosed in a globe which is almost air tight, 
so that after the first few seconds the arc is operating in an atmos- 
phere of carbonic acid gas, and the carbons are consumed more 
slowly than in an open arc and have a life of about 100 hours. 
The arc is operating under slight pressure and, for satisfactory 
operation, the arc stream is longer than that of the open arc. 



356 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xlii 

The crater is not now so pronounced, and a greater portion of the 
total light now comes from the arc stream, so that the light dis- 
tribution in a horizontal direction is improved. Data on this 
arc is given in the table on page 360. 

385. Alternating-current Enclosed Arc. — When operating 
with alternating current, the carbon arc lamp does not go out 
when the current passes through the zero value, because there 
is enough heat in the carbon tips to maintain the arc stream while 
the current reverses. Alternating-current arcs have no crater, 
and each tip is equally hot but is not so hot as the crater of a 
direct-current arc, so that the efficiency is lower, moreover the 
light is not directed downward but is directed horizontally, so 
that a reflector has to be supplied to deflect the light in the 
direction shown in Fig. 424. 




Fig. 424. — Light distribution of arc lamps. 



Fig. 425.— Shape of 
direct-current arc. 



386. Flame Arc Lamps. — The positive carbon of the direct- 
current flame arc lamp and either or both carbons of the alter- 
nating current flame arc are impregnated with salts which have 
high selective radiation. Such salts when heated give most of 
their radiation in one particular part of the spectrum. Being 
selective, the portion of the total radiation which is given off as 
light is greater than that given off by an ordinary incandescent 
body at the same temperature, such lamps are therefore very 
efficient, and a large portion of the light comes from the arc 
stream. Calcium salts are often used and give a yellow light. 
Barium salts give a white light, but the white flame arc is not so 
efficient as the yellow flame arc. 



Art. 388] ELECTRIC LIGHTING 357 

The open flame arc requires daily trimming. If the arc is 
enclosed so as to limit the supply of air, the life of the carbons may 
be increased to 100 hours without any marked reduction in the 
efficiency. The enclosing globe, however, must be kept free 
from soot, by arranging that the fumes shall be carried away and 
allowed to deposit in a condensing chamber and not on the sides 
of the enclosing globe. 

387. Luminous Arc Lamp. — This is a low temperature arc 
which depends entirely on selective radiation for its efficiency. 
It is essentially a direct-current arc and has a positive electrode 
of copper and a negative electrode of magnetite. Magnetite 
boils at a much lower temperature than carbon, and the tempera- 
ture of the arc is not high enough to melt the copper or to enable 
the arc to be used on alternating current; the arc goes out as the 
current passes through zero. 

The efficiency is not so high as that of the flame arc. The light 
is white, and the magnetite electrode has a life of about 150 hours 
while that of the copper electrode exceeds 1000 hours. The mag- 
netite arc must never be connected with the polarity reversed 
because, if the copper were made the negative electrode, a copper 
arc would be produced instead of a magnetite arc, since the mate- 
rial of the arc stream comes from the negative electrode. 

For alternating- current circuits, titanium arcs are being devel- 
oped, which have an efficiency comparable with that of the flame 
arc. 

388. Mercury Vapor Converter. — To obtain direct-current 
from an alternating- current supply for the operation of magnetite 
arcs, the mercury vapor converter is used. 

The globe shown in Fig. 426 is filled with mercury vapor. It 
is found that a very high voltage is required to start an arc 
between a and b but about 14 volts is all that is required to main- 
tain the arc ; there is a high resistance at the negative electrode 
which resistance is broken down when current is flowing, but 
is reestablished as soon as the current ceases to flow. Alter- 
nating current cannot pass between the electrodes because the 
high resistance is reestablished as the current passes through the 
zero value. 

To operate as a converter, the globe is fitted with three elec- 
trodes and is connected up as shown in Fig. 428, and means are 
provided whereby the resistance of electrode c is kept broken 
down. At the instant shown in Fig. 428, the electrode a is posi- 



358 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap.xlii 

tive and that of b is negative, so that current passes from a to 
c but no current can pass from a to b or from c to ?>, because of the 
high resistance of negative electrode b. Half a cycle later b is 
positive and a is negative, so that current can pass from b to c but 
no current can enter a. The current in the line L therefore flows 
always in one direction or is a direct current. 

There is a small reactance coil S, called a sustaining coil, 
placed in the line L to carry the rectifier over the point of zero 
current. This coil causes the current to lag slightly behind the 
e.m.f. so that there is still a small current flowing from a, for exam- 



Single Phase Mains 
Eeactance Coil 



^fmmjmm^- 





Single Phase Mains 



Eeactance Coil 



-nrn^w^- 




Fig. 426. Fig. 427. Fig. 428. 

Figs. 426 to 428. — The mercury-vapor converter. 



pie, when the arc from b is ready to strike. The current entering 
c therefore never becomes zero and the high negative resistance of 
that electrode is always broken down. 

Before such a converter can be started, the resistance at c must 
first be broken down. To accomplish this result an additional 
starting electrode is placed at w and current is passed between 
w and c by tipping the globe until the mercury forms a bridge 
between these two electrodes. The tube is then raised and an 
arc is drawn between w and c for half a cycle, which breaks down 
the resistance of c long enough to allow the arc to start from b 
and thereby start the rectifier in operation. 



Art. 390] 



ELECTRIC LIGHTING 



359 



389. Mercury Vapor Lamp. — The converter described above 
is an arc lamp and, since the light is due to the selective radiation 
of mercury vapor, it is a high efficiency lamp. Unfortunately 
the color is greenish blue and gives ghastly color effects. 



ngle Phase 
Mains 




Fig. 429. — Mercury-vapour lamp for operation by alternating-current. 

The alternating-current lamp is made in the form shown in 
Fig. 429, which is lettered similarly to Fig. 427. To start the arc, 
the lamp is tipped so that the 
mercury runs down and forms a 
metallic connection between the 
two electrodes through which a 
current flows, the lamp is then 
allowed to return to its original 
position, the mercury thread is 
ruptured, and an arc follows. 

The direct- current lamp is 
supplied with two terminals and 
is started in the same way. 

When a quartz tube is used 
instead of a glass tube, the 
lamp may be shortened, and run 
at a higher temperature with a 
higher efficiency. 

390. Shades and Reflectors. 
— The light distribution from a 
source can be completely changed 
by the use of a shade or reflec- 
tor. Curve A, Fig. 430, shows the light distribution from a tung- 
sten lamp while curves B, C and D show the distribution when 




D Focusing 
Reflector 



Fig. 430.— Light distribution of 
a 32 candle-power tungsten lamp 
equipped with different reflectors. 



360 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xlii 



different types of reflectors are used. Curve B is obtained with 
what is called an extensive reflector, curve C with an intensive 
reflector, and curve D with a focusing reflector. 

When the distance between the lamps is at least twice the height 
of the lamp above the ground, the extensive type is used; it is 
suitable for a small room lit by a source placed on the centre of 
the ceiling. When the distance between lamps is about one and 
a half times the mounting height, the intensive type is preferred; 
it is suitable for a large room lit from several points on the ceiling. 
The focusing type is used when the lamps are more closely spaced 
and is suitable for the illumination of desks, show cases, etc. 

391. Efficiency of Illuminants. — Since the light from a source 
can be suitably directed by means of reflectors, light efficiency 
should be based on the mean spherical candle power of the lamp. 
Values attained in practice are given below 1 





Mean 
spherical 

candle 
power per 

watt 


Candle 
power at 


Available 
size in 
mean 




10° angle 
per watt 


spherical 
candle 
power 


Incandescent lamps 








Ordinary carbon filament 


0.21 


0.4 


a 113' size 


Tungsten filament 


0.64 


1.25 


any size 


Gas-filled tungsten 


1.28 


2.5 


above 350 


Arc lamps 








Enclosed carbon 








6.6 amp., 450 watts A.C. 


0.39 


0.5 


175 


6.6 amp., 480 watts D.C. 


0.62 


1.0 


300 


I lame carbon 500 watts, yellow 


3.1 


6.2 


1550 


300 watts, yellow 


1.95 


4.0 


585 


500 watts, white 


1.95 


4.0 


975 


D.C. magnetite 4 amp., 300 watts 


1.0 


2.2 


300 


6.6 amp., 500 watts 


1.5 


3.2 


750 


A.C. titanium 220 watts 


1.9 


4.0 


420 


Mercury lamps glass tube 


1.55 






quartz tube 


2.0 







These figures take account of the loss in the reflectors, and the 
candle power at 10° angle below the horizontal is that obtained 
when the lamp is equipped with a reflector suitable for street 
lighting. 

1 Efficiency of Illuminants by C. P. Steinmetz, General Electric Review, 
March, 1914. 



Art. 392] 



ELECTRIC LIGHTING 



361 



392. Light and Sensation. — The eye is able to see objects 
distinctly over a range of intensity of 1,000,000 to 1 as determined 
by the exposure of a photographic plate. The size of the pupil is 
controlled automatically by the intensity of the light and decreases 
as the light intensity increases so as to limit the amount of light 
that can enter the eye. The sensibility of the optic nerve also 
changes automatically with the light intensity but at a much 
slower rate; when one goes from daylight into a darkened room, 
for example, objects that at first are invisible become quite 
distinct after a time. 




Ultra Red 



I, Ultra Violet 



Fig. 431.— Effect of color "on the light efficiency. 

The light sensation produced by a given amount of radiant 
power entering the eye depends on the color of the light. White 
light is composite and, when passed through a prism, as shown in 
Fig. 431, is divided up into its constituents and produces a band 
of color varying from red to violet called the spectrum. 

The relation between candle-power per watt and color is 
shown in Fig. 431; with a high light intensity the eye is most- 
sensitive to yellow light while with a low intensity it is most sen- 
sitive to green light. If for example a mercury vapor lamp and 
a yellow flame arc burning side by side have the same brightness 
at a moderate distance from the observer, then, when the lamps 
are close at hand the flame arc appears the brighter since the light 

24 



362 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xlii 

intensity is high, but when the lamps are a considerable distance 
away the intensity is low and the green mercury lamp appears 
the brighter. A yellow light is therefore the most efficient for 
high intensity illumination and a green light the most efficient 
for low intensities ; these colors however are often objectionable. 
There is one important exception; yellow light is used for fog 
signal work because it penetrates fog better than does green light. 

393. Reflection and Color. — When light strikes an opaque 
body, some of the light is absorbed and some is reflected. The 
color of an opaque body is the color of the light which it reflects, 
a green opaque body is one which absorbs all but the green rays. 

When light strikes a transparent body, the color of the body as 
seen from the side away from the source is the color of the light 
which is transmitted; a green glass transmits only the green rays, 
whereas a transparent body transmits all the light that falls on it, 
a colored globe on a lamp therefore cuts off light and reduces the 
efficiency. 

The color of a body depends on the color of the light that 
strikes on it. An opaque body which is yellow in daylight reflects 
only yellow rays ; in the green light of the mercury vapor lamp 
this body looks black because it absorbs the green light and there 
is no yellow to reflect. It is important therefore that colors be 
matched in the light in which they are going to be used. 

The color of the light in a room is not always the color of the 
source. If daylight enters a room which has walls of a green color, 
then the bulk of the light that strikes the walls is absorbed while 
the green rays are reflected and a green tint is added to the light 
in the room. Dark walls and ceilings result in low efficiency of 
illumination. In an ordinary room, about 70 per cent, of the 
light from the source strikes the walls and, if the paper is dark, 
most of this light is absorbed and lost, if the paper is light yellow 
in color, a large part of the light is reflected and is useful. 

Red wall papers and table cloths are to be avoided in a reading 
room because, under such circumstances, a large part of the light 
that enters the eye is red light and, for a given distinctness of 
vision, a larger amount of energy must enter the eye if the light 
is red than if yellow or white, see Fig. 431, page 361, and the 
excessive amount of energy is harmful to the eye tissues. 

394. Principles of Illumination. — For satisfactory illumination, 
the light should be of good quality, glare should be avoided, and 
the shadows should be distinct so as to give good perspective. 



Art. 397] ELECTRIC LIGHTING 363 

395. Quality of the Light. — The light should be as nearly white 
as possible. Most of the harm done by artificial illumination is 
due to the red and ultra red rays for the reason just pointed out. 
Green light also is harmful under certain circumstances because 
it is found that the pupil fails to respond to variations in intensity 
of light of this color. Green light should therefore not be used 
for high intensity illumination, as for example for the lighting of 
drawing offices, but, as pointed out on page 362, it is particularly 
suited for low intensity illumination. Ultra red and ultra violet 
radiation is the most harmful and there is much of the latter in 
arc lamps, so that an arc should always be enclosed in a glass 
globe since glass is opaque to such radiation. 

Flickering light is bad for the eye because the pupil tries to 
adjust itself to the rapidly varying intensity and becomes fatigued. 
Alternating current lighting at 25 cycles has not been uniformly 
successful for this reason. 

396. Glare. — The eye cannot look with comfort on objects 
which have a higher surface intensity than 4 candle power per 
square inch, so that the sources of illumination should be kept 
out of the range of vision, and direct reflection into the eye from 
such sources should also be avoided. Side lights and light from 
below are particularly objectionable since the eye is not protected 
from such light. 

If the source of illumination cannot be kept out of the range 
of vision, then frosted incandescent lamps should be used or the 
lamp supplied with a shade or globe that will keep the direct 
rays away from the eye. 

It is impossible to see distinctly past a bright light and for 
that reason country roads should not be lit by powerful arc lamps 
spaced far apart because, not only does most of the light go to 
illuminate the adjoining fields, but the arcs are generally hung 
so low to clear the trees that it is impossible to see past them, and 
driving on such roads is dangerous. Incandescent lamps of 
moderate candle power, spaced closer together, give better illu- 
mination. 

397. Shadows. — In certain cases as for example for the illu- 
mination of drafting rooms it is desirable to eliminate shadows. 
This result is obtained by the use of a large number of light 
sources, or by the use of indirect methods of lighting wherebj^ 
a [reflector is used to throw all the light on to the ceiling from 
which it is reflected on to the drawing tables. 



364 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xlii 

For other classes of work, such as street lighting, diffusion is to 
be avoided since, due to the elimination of shadows, there is loss 
of perspective and obstacles are not clearly seen. 

398. Intensity of Illumination. — The unit of light intensity 
is that produced by a source of one candle-power at a distance of 
1 ft. and is called the foot-candle. 

In Fig. 432, the same amount of light strikes the surfaces 
A, B and C, so that the intensity at B is less than that at A in 
the ratio a r 2 /b r 2 , or intensity is inversely proportional to the 
square of the distance. Again the intensity on surface C is less 
than that at B in the ratio area B/ area C = cos a. 



40 50 Candle Power 




Fig. 432. — Variations of light intensity with the distance from the source, 
and with the angle of incidence. 



Curve D, Fig. 432, shows the light distribution curve of a 40-candle- 
power tungsten lamp equipped with a reflector for street lighting. It is re- 
quired to determine the illumination at a point X, 50 ft. from the post, the 
height of the lamp being 12 ft, 

The candle-power in direction OX = 51 

The distance OX = V50 2 + 12 2 = 51.5 ft. 

The intensity on an obstacle normal to the light 

= JeN, = 0.0192ft.-candle 
(51.5) 2 

The intensity on the street surface = 0.0192 X cos a 

12 



0.0192 X 



51.5 



0.0045 ft.-candles. 



The minimum intensity on the street surface due to two lamps spaced 
100 ft. apart = 2 X 0.0045 = 0.009 ft.-candles. For country roads, the 
minimum intensity on an obstacle normal to the light should not be less 
than 0.02 ft.-candles. 

399. Lines of Illumination. — It is convenient to represent the 
total light from a source by lines of illumination, called lumens, 



Art. 400] ELECTRIC LIGHTING 365 

such that the number crossing unit area placed perpendicular to 
the direction of the light is made proportional to the light in- 
tensity. One foot-candle is represented by one- line per sq. ft. 

If a source of one candle-power were surrounded by a sphere 
of 1 ft. radius, the intensity at the surface of the sphere 
would be 1 ft.-candle, there must therefore be one line per sq. 
ft. or a total of 4x lines. 

To provide for a surface intensity of I ft.-candles over an area 
of A sq. ft. 

The number of lines of illumination = I X A 

I X A 

The candle-power of the source = — j- — 

Since some of the light is absorbed by walls and ceilings 
the necessary candle-power = j — — r- 

where k = , ,. , — is less than 1 ; average values are 

total light 

k — 0.6 if clear reflectors are used and the room has light 

walls and ceiling. 

= 0.4 with clear reflectors and dark walls and ceiling. 

Determine the candle-power required to give a light intensity of 3 ft.- 
candles over a room which is 40 ft. long and 30 ft. wide the room having 
light walls and ceiling. 1 

3 X 40 X 30 at* J! * ,n 

The necessary candle-power = . . 7 — = 477 candle-power. A 40- 

watt lamp gives 40/1.25 = 32 horizontal candle-power and 24 mean 

477 
spherical candle-power, see page 354, so that -~-r = 20 lamps of 40 watts 

each are required, or a smaller number of lamps of larger candle-power. The 
choice of the candle-power of each lamp, and the spacing of the lamps, is a 
matter that must be left to the judgment of the individual; there are many 
examples of good and of bad illumination to be found in every city. 
In the above problem 

The total power required = 20 X 40 = 800 watts 

The watts per sq. ft. = 800/(40 X 30) = 0.75. 

400. Power Distribution for Lighting. — Interior lighting is 
generally carried out at 110 volts, either alternating or direct 
current, the lamps being connected in parallel across the circuit. 
When arc lamps are connected across such a circuit, a steady- 
ing resistance must be placed in series with the arc as shown in Fig. 
433. Suppose an arc is carrying a current / with a constant 

1 A list of economical intensities for all classes of work will be found in 
the American Electricians' Handbook by Terrell Croft. 



366 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xlii 

voltage E at the terminals. If the current I were to increase for 
an instant, more carbon vapor would leave the negative electrode, 
the arc stream would become more conducting, and the current 
/ would increase still further, so that an arc is unstable when used 



m 



E 

Constant 



VI 



E 

Constant 




Fig. 433. — Direct-current. Fig. 434. — Alternating-current. 

Figs. 433 and 434.— Multiple arcs. 

on constant potential. If, however, a resistance is placed in 
series with the arc, as shown in Fig. 433, then an increase in the 
current I causes the voltage Ex = IR to increase, and therefore 
the arc voltage E 2 to decrease so that it cannot maintain the in- 
creased current across the arc. 



VWIM 



&-Q-&-£ 



Constant Current 
Transformer 



Fig. 435. — Alternating-current system. 



^i 




Mercury Vapour 
Converter 

Fig. 436. — Direct-current system. 
Series connection of lamps. 

In the case of alternating- current parallel arcs a steadying 
reactance is used, because it consumes very little power and still 
reduces the voltage. The same result may be obtained by sup- 
plying the lamp through an autotransformer since the sec- 
ondary voltage E 2) Fig. 434, decreases with an increase of the arc 
current 7 2 . 

If the lamps can be used in a series circuit, as shown in Fig. 
435, then the wire has to carry the current of only one lamp and 



Art. 400] ELECTRIC LIGHTING 367 

may be small in cross section. This system of distribution is 
largely used for street lighting, the constant current required for 
the operation of the arcs being obtained by means of a constant 
current transformer, see page 267. 

By means of a series transformer s it is possible to connect a 
circuit of tungsten lamps taking a large current in series with a 
main circuit of arc lamps taking a smaller current. 

For the operation of magnetite arcs, direct current is necessary 
and this is obtained by means of a mercury vapor converter con- 
nected as shown in Fig. 436. 

The voltage between electrodes should be about: 

47 volts for an open carbon arc, 

72 volts for an enclosed carbon arc, 

45 volts for an open flame arc, 

78 volts for open magnetite arc. 



CHAPTER XLIII 
LABORATORY COURSE 

401. Protection of Circuits. — A typical circuit is shown in 
Fig. 437. The lamps L take power from the mains and the 
current I is measured by the ammeter A while the voltage E is 
measured by the voltmeter V, see page 19. 

All the connections should be made before the switch S is 
closed, and the circuit must be protected by fuses F, see page 117, 
or by an automatic circuit breaker, see page 39, set so as to 
open the circuit if the current should become large enough to 
injure any of the apparatus connected in the circuit. 

To make sure that the instruments are reading in the proper 
direction, the switch S should be closed for an instant and then 



f . 



Ammeter 
E Qy Shunt 



Fig. 437. — Typical circuit. 

opened before any appreciable current has had time to flow. 
If the needle moves in the wrong direction, the instrument 
connections must be reversed. 

402. Ammeter Shunts. — Most ammeters are wound with 
very fine wire, see page 8, and can carry only a small fraction 
of an ampere without being burnt- out. To use such instruments 
for the measurement of large currents they must be connected 
in parallel with shunts as shown in Fig. 437. The current in 
the instrument, and therefore the deflection, will be proportional to 
the line current I so that, if the shunt and instrument are always 
used together, the scale of the instrument may be in terms of the 
line current to be measured and not in terms of the current in the 
instrument. For instruments with a range of less than 5 am- 
peres, the shunt is generally placed inside the case. 

403. Safe Carrying Capacity of Copper Wires. — If too large 
a current flows in an insulated wire, the insulation will be 

368 



Art. 404] 



LABORATORY COURSE 



369 



damaged. The values given in the following table should not 
be exceeded. 



Size of wire, Brown 
& Sharpe gauge 
number 


Diameter of wire 
in inches 


Maximum current in the wire in 
amperes 


Rubber Other 
insulation insulation 


14 


0.064 


12 16 


12 


0.081 


17 23 


10 


0.102 


24 32 


8 


0.12.8 


33 46 


6 


0.162 


46 65 


5 


0.182 


54 77 


4 


0.204 


65 92 


3 


0.229 


76 110 


2 


0.258 


90 131 


1 


0.289 


107 156 





0.325 


127 185 



404. Control of the Current in a Circuit. — To vary the current 
flowing in the coil C, Fig. 438, an adjustable resistance R is 
inserted in the circuit, see page 25. This rheostat must be 
able to carry the current I without overheating. 



— ^--AA/VAVv 




Figs. 



Fig. 438. Fig. 439. 

438 and 439. — Methods of controlling the current in a circuit. 



If the resistance of the coil C is large compared with that of 
the resistance R, then the current variation w T ill not be very 
large. Under such circumstances, the potentiometer connection 
shown in Fig. 439 is to be preferred for experimental work. If 
the movable contact a is placed at b then the voltage E x is equal 
to E, the line voltage. If contact a is placed at x, then the 
voltage E 1 is zero. By moving the contact between these two 
points, the voltage E\ and the current I may have any value from 
zero to Ei = E and I = E/R c . If the resistance R c is small and 
the contact a is close to b, as shown in Fig. 439, then the current 
in ab may become dangerously large, so that the rheostat must 



370 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xliii 

be watched during operation and the circuit opened if the 
rheostat becomes too hot. 

EXPERIMENT 1 

Object of Experiment. — To determine the resistance of the 
shunt field circuit of a direct-current machine with different 
values of current in the circuit. 

Reference. — Pages 20 and 95. 

Connections. — If the connection in Fig. 440 does not give 
sufficient range of current, then use the connection shown in 
Fig. 439. 

Readings. — Volts and amperes. 

Report. — Describe the method of 
___ ywwvw ^ e ' carrying out the experiment and embody 

i ia the answers to the following questions in 

S3Field & ^ 

coiia the report. ; 

1. Find the resistance of the field coil 



Voltmeter (M 



circuit. 
Fig. 440. 2. How does temperature affect this 

resistance? Show the result experi- 
mentally by measuring the resistance after the current has 
been flowing for 30 minutes. 

3. What per cent, of the output of the machine is the excitation 
loss? The machine output is given on the name plate. 

4. What range of instruments would you use to determine 
the resistance of the shunt field circuit of a 50 kw., 240 volt 
generator if the excitation loss is known to be. less- than 4 per 
cent.? 

EXPERIMENT 2 

Object of Experiment. — To determine the resistance of the 
armature circuit of a direct- current machine with different values 
of current in the circuit. 

Connections. — See. Fig. 438, page 369. The rheostat used 
to control the current must be able to carry the full-load armature 
current of the machine without injury. 

Readings. — Volts across the terminals; volts across the com- 
mutator, obtained by attaching the voltmeter leads to the seg- 
ments which are under the brushes; amperes. 

Report. — Describe the method of carrying out the experiment 
and embody the answers to the following questions in the report: 



Art. 404] LABORATORY COURSE 371 

1. Plot the armature resistance against current. 

2. Plot the brush contact resistance against current. 

3. What per cent, of the output of the machine are the armature 
resistance loss and the brush contact resistance loss at full-load? 

4. Has the pressure on the brushes much effect on the brush 
contact resistance? try the experiment. What do you imagine 
limits the brush pressure? 

5. What range of instruments would you use to determine the 
resistance of the armature circuit of a 50-kw., 240- volt generator if 
the loss in the complete armature circuit is known to be less than 
4 per cent, at full-load ? 

EXPERIMENT 3 

Object of Experiment. — To find how the speed of a direct 
current shunt motor at no-load varies with: 

a. The exciting current; armature voltage being constant. 

b. The armature voltage; exciting cur- 
rent being constant. 

References. — Pages 85 and 89. 

Connection. Pig. 441. 

Readings. — Exciting current; arma- 
ture voltage; speed. p IG 441 

Curves a. Speed and exciting current. 
b. Speed and armature voltage. 

Questions. — 1. Explain the shape of the curves, without 
formulae 

2. What would happen if, in starting up a shunt motor, a 
starting resistance was not placed in series with the armature 
circuit? How many times full-load current would flow through 
the armature under these conditions? (Use the value of armature 
resistance determined in the last experiment.) 

3. What is the back e.m.f . of a motor and what relation has it to 
the applied e.m.f.? Perform the experiment described in Art. 
88, page 79, to answer this question. 

4. What would happen if, during operation, the field coil 
circuit were to open (do not perform this experiment) ? 

5. Draw a diagram of connections of any starter in the labo- 
ratory, which has a no-voltage release and also overload pro- 
tection. 

EXPERIMENT 4 

Object of Experiment. — To find how the voltage of a direct- 
current generator at no-load varies with: 




372 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xliii 

a. The exciting current; speed being constant. 

b. The speed; exciting current being constant. 
Reference. — Page 70. 

Connection. — See Fig. 95, page 70. 

Readings. — Voltage, exciting current and speed. 

Curves a. Voltage and exciting current. 

b. Voltage and speed. 
Questions. — 1. Why is there a small voltage even with no 
exciting current? 

2. Explain the shape of the curves. 

3. How would you reverse the polarity of the generator, i,e., 
how reverse the direction of the voltage? 

EXPERIMENT 5 

Object of Experiment. — To determine how the terminal volt- 
age of a constant speed generator varies with the load : 

a. The generator being separately excited; 

b. The generator being shunt excited; 

c. The generator being compound excited. 
References. — Pages 71 to 77. 

Connections.— See Fig. 97, page 72; Fig. 98, Fig. 99. 

Readings. — Terminal voltage, line current, exciting current 
and speed. 

Curves. — Terminal voltage on a line current base. 

Shunt exciting current on a line current base. 

Questions. — 1. Why does the terminal voltage of a separately 
excited generator decrease with increase of load? How much of 
the drop at full-load is due to armature resistance? 

2. Why is the voltage drop greater in a shunt than in a sepa- 
rately excited generator? 

3. What would be the effect of shifting the brushes further for- 
ward from the neutral position? 

4. What is the principal advantage of the compound generator? 

5. How would the terminal voltage of a compound generator 
vary with increase of load if the series field coils were connected 
so as to oppose the shunt coils? 

6. If the generator is overcompounded while flat compounding 
is desired, what can be done to fix the machine? 

7. If the voltage of a shunt generator builds up when the 
generator rotates in a given direction, why will it not build up if 
the direction of rotation is reversed? Try the experiment. 



Art. 404] LABORATORY COURSE 373 

EXPERIMENT 6 

Object of Experiment. — -To determine the efficiency and also 
the speed and torque characteristics of shunt, series, and com- 
pound motors, by loading the machines by means of a brake, 
the applied voltage being constant. 

References.— Chaps. XV, XVII and XVIII. 

Connections.— Fig. 107, page 86, Fig. 113 and Fig. 117. 

Readings. — Applied voltage (constant), armature current, 
shunt exciting current (constant), speed, brake reading. 

Curves. — Speed and torque on an armature current base. 
Brake horse-power and total input on an armature current base. 
Efficiency on a brake horse- power base. 

Questions.— 1. How would you reverse the direction of 
rotation of each machine? 

2. For what type of service is each machine suited? 

3. The field coils of a machine become hot during operation, 
what effect will this have on the no-load and on the full-load 
speed of each type of motor? 

4. How can the speed of each type of motor be varied for a 
given load? 

5. Why is the speed regulation of a shunt motor poor when 
the speed is controlled by a resistance in the armature circuit? 
Try the experiment. 

6. Explain without formulae, the shapes of the speed, torque, 
and efficiency curves. 

EXPERIMENT 7 

Object of Experiment. — To determine the relation between 
starting torque and armature current in a shunt, a series, and a 
compound motor. 

References. — Pages 85 and 90. 

Connections. — Same as for experiment 6, with resistance in 
the armature circuit to limit the current (do not use a starting 
box for this purpose). 

Readings. — Armature current and brake reading. 

Curves. — Torque on an armature current base. 

Questions. — 1. How does the starting torque compare with 
the running torque, as determined in experiment 6, for the same 
armature current? Does theory indicate that there should be a 
difference? 

2 r Why is the series motor preferred for heavy starting duty? 



374 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xliii 

EXPERIMENT 8 

Object of Experiment. — To measure the stray loss, the 
armature copper loss, and the excitation loss in a shunt motor, 
and calculate the efficiency from these figures. 

Reference— Chap. XVI, page 95. 

The work of this experiment should be done without instruc- 
tion. A diagram of connections should be drawn out and the 
range of the necessary instruments determined before the 
apparatus is connected up. 

Report. — Describe the method of carrying out the experi- 
ment. Plot the efficiency curve on a horse-power output base 
up to 25 per cent, overload, and compare this curve with that 
obtained by brake readings in experiment 6; if the results show 
considerable difference, which would you consider to be the more 
reliable? 

EXPERIMENT 9 

Object of Experiment. To run a shunt generator in parallel 
with the power house and determine the temperature rise of the 
machine at full-load. 

References. — Pages 21, 99 and 163. 

Connection. — Fig. 179, page 164. 

Readings. — Measure the resistance of the field coil circuit 
at the beginning of the test and every ten minutes thereafter, take 
also readings of temperature of the field coil surface at the same 
time. Find the temperature of the armature core and armature 
winding immediately the generator is shut down; the heat run 
should last for at least an hour and a half. 

Curves. — Observed temperature rise of field coil surface, and 
also the temperature rise determined by resistance measurements, 
on a time base. 

Questions. — 1. Explain the shape of the curves. 

2. Why is the temperature rise of the field coils as determined 
by resistance measurements greater than that determined by 
thermometer? 

EXPERIMENT 10 

Object of Experiment. — To determine the voltage regulation 
of a three wire system. 

References.— Pages 316, 317 and 338. 

Connections. — Use a balancer set, see Fig. 374, or a three- wire 



Art. 404] 



LABORATORY COURSE 



375 



generator. If these are not available, a single-phase rotary 
converter with a suitable autotransformer may be connected 
up as shown in Fig. 442 to form a three-wire generator. 

Readings. — Vary the loads on the two sides of the system from 
perfect balance, when A\ = A 2 and A n is zero, to a maximum 
of unbalance when the load on one side of the line is zero. Take 
readings of V h V 2 , A h and A 2 ; Vi + V 2 to be kept constant. 

Curves. — Plot V\ and V 2 against 
the unbalanced current Ai-A 2 . 

Questions.— 1. Explain the action 
of the apparatus used. 

2. What are the advantages of the 
three-wire over the two- wire system 
of distribution? a* 

3. Explain the shapes of the curves. Fig. 442. 



* — 




Rotary 
Converter 



Fi+Vo 



EXPERIMENT 11 



/ w wv^ 



Fuse Wire 

— 3— 



-a 



Fig. 443. 



Object of Experiment. — To determine the characteristics of 
fuse wire. 

Reference. Page 117. 
Connection. Fig. 443. 

Readings. — Length of wire between blocks, average current, 
time taken for fuse to melt after switch S is closed. 

Curves. — Plot amperes on a time 
base forf our lengths of fuse wire and 
from these curves plot another set 
with amperes on a length base for a 
fusing time of 10 sec. 

Questions. — 1. Explain the shape 
of the curves. 

2. If a fuse is rated at 5 amp., about what current would you 
expect it to carry continuously, and what current for 30 sec, ? 

EXPERIMENT 12 

Object of Experiment. — To calibrate a circuit breaker. 
Reference. Page 39. 
Connections. — Same as for fuse testing. 
Readings. — Amperes to open, and position of plunger. 
Curve. — Draw a current scale that could be attached to the 
circuit breaker. 



376 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xliii 

Questions. — 1. Explain the construction and the operation of 
the circuit breaker used. 

2. What are the advantages and disadvantages of circuit 
breakers and fuses, for what type of circuit is each suited? 

.3. If a 10 amp. fuse, and a circuit breaker set for 15 amp., 
are used to protect the same circuit, which would open first in 
the case of an overload on the circuit. 

4. If a circuit breaker and a switch are both in a circuit, as in 
Fig. 417, page 349, which would you close first? 

EXPERIMENT 13 
Object of Experiment. — To determine the effect of change of 
frequenc}^ on the constants of an alternating- current circuit. 
Reference. Chaps. XXIX and XXX. 
Connection. Fig. 444. 

Readings. — Vary the alternator speed and adjust the alter- 
nator excitation to keep E t con- 
stant. Take readings of E r , E h 
E c , I, watts, and speed. Measure 
the resistance R and the resistance 
of the coil Xi with direct current. 

Curves. — On a frequency base, 
plot the inductive reactance Xi = 
Ei/I, the capacity reactance X 2 = E c /I, the resistance R, 

I W 
and the power factor ( ^r 

Questions. — 1. For any one frequency, draw the voltage vector 
diagram to scale, and compare the value of E t so found with that 
found experimentally. 

2. Calculate the coefficient of self induction of the coil Xi and 
the capacity of the condenser X 2 . 

3. What is meant by resonance? At what frequency does it 
occur in this experiment and how does that frequency compare 

with the theoretical value/ = -. j=--1 

^ -ylLC 

4. Explain the shape of the power factor curve. 

5. Under what conditions can the voltage E t be greater than 
the applied voltage E t ? 

EXPERIMENT 14 

Object of Experiment. — To predetermine the characteristics 
of a given circuit and compare the results with those obtained by 
actual test. 




444. 



Art. 404] LABORATORY COURSE 377 

Reference. Chaps. XXIX and XXX. 

Connection. — Use the same resistance, inductance, and capacity 
as in the last experiment and connect as in Fig. 445. 

Curves. — Predetermine the values of E r , E x , Ii, I c and I, 
with E t , the normal voltage of the alternator, the same as in the 
last experiment. Plot these values 
against frequency, then determine the 
same curves by test and compare the 
results. 



A'i 



EXPERIMENT 15 



1 



R I 



O5000 

h 



-EV 



^H 



*w 



Fig. 445. 




Object of Experiment. — To deter- . 
mine the characteristics of a transformer. 
References. Chap. XXXIV, page 261. 

Connections. — A low voltage transformer should be used, con- 
nected as shown in Fig. 446, or else two like transformers con- 
nected as shown in Fig. 447 with the high- voltage leads taped up. 
Readings. — Keep E h the secondary power factor W 2 /E 2 I 2 , 
h W2 and the frequency all constant, and 

~ take readings of E h I h W h E 2 , I 2 , and 
- W 2 for values of I 2 from zero up to 25 
per cent, over-load. Measure also the 
resistances of the primary and the sec- 
ondary windings with direct current. 

Curves. — Plot efficiency {W 2 /Wi), voltage V 2 , and primary and 
secondary power factors against I 2 . 

Questions. — 1. Why is the primary power factor less than that 
of the secondary particularly at light loads? 

2. Why does the voltage E 2 decrease with increase of load, and 
why is the voltage drop greater with inductive than with non- 
inductive load? 

3. Calculate the resistance loss in the windings at full-load. 
Find also by calculation the full-load efficiency and compare the 
result with that determined by test. 

4. If the transformer is connected to the line for 24 hours a 
day, but carries full-load for only 5 hours a day, find the all day 
efficiency. 

EXPERIMENT 16 

Object of Experiment. — To predetermine the regulation curves 
of a single-phase alternator at 100 per cent., 80 per cent, and 

25 



378 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xliii 

zero power factors, from no-load saturation and short-circuit 
curves, and compare the result at 100 per cent, power factor 
with that found by actual load test. (A lamp bank has a power 
factor of approximately 100 per cent.) 

Reference. XXXII, page 244. 

Connections. Pig. 287, page 247. 

Readings. — a. No-load saturation; armature volts and field 
current at constant speed. 

b. Short-circuit; armature amperes and field current at the 
same speed. 

c. Load curve; terminal voltage and armature current with 
constant field excitation and the same constant speed. 

d. Measure the armature resistance with direct current. 
Curves. — a. No-load saturation; armature voltage on a field 

current base. 

b. Short circuit; armature current on a field current base. 

c. Armature reactance determined from curves a and b plotted 
on a field current base. 

d. Load curve; terminal voltage on an armature current base, 
by calculation at 100 per cent., 80 per cent, and zero power 
factors, and by test at 100 per cent, power factor. 

Questions. — 1. Why is the power factor of a bank of in- 
candescent lamps approximately equal to 100 per cent. 

2. Give the theory of the method used to determine the re- 
actance of the armature of the alternator. 

3. Why does the voltage drop more rapidly at low power factors 
than at high power factors? 

4. Why are alternators rated in kilo volt-amperes and not in 
kilowatts ? 

5. How is the voltage of an alternator maintained constant in 
practice, at all loads and power factors? 

EXPERIMENT 17 

Object of Experiment. — To start up a synchronous motor and 
determine its running characteristics. 

Reference. Chap. XXXIII, page 252, also page 296. 

Connections. — Fig. 398, page 258, for single phase machines. 
For three phase machines use the connection in Fig. 448. 

Method of Starting.— Start up the synchronous motor by 
means of the belted direct- current motor M and adjust the speed 



Art. 404] 



LABORATORY COURSE 



379 



of the synchronous motor and its excitation until the voltage E 2 
is equal to Ei and the synchronizing lamps all remain dark for a 
few seconds at a time. (If all the lamps do not become dark at 
the same instant then interchange two of the motor leads.) 

When all three lamps are dark at the same instant, then close 
the three switches Si S 2 and $ 3 at the same time. The starting 
motor can then be disconnected by throwing the belt, and the 
synchronous motor loaded by means of a prony brake. 

Readings. — With the applied voltage E 2 , the frequency, and 
the brake readings all constant; take readings of current / and of 
watts (TTi+ W 2 ) for different values of the exciting current If and 
for three different settings of the brake. 

Synchronizing 
Lamps 

Alternator/ | <£>i 




Starting 
Motor 



Fig. 448. 



Curves- 
watts 



-Plot armature current, and power factor 



\ 



on an exciting current base. 



Vvolt amperes/ ' 
Questions. — 1. Explain the shapes of the curves. 

2. What are the advantages and disadvantages of the syn- 
chronous motor compared with the induction motor? 

3. What effect has the power factor of the load on the size of 
the alternator and on the power loss in the transmission line supply- 
ing the load? 

EXPERIMENT 18 

Object of Experiment/ — To determine the characteristics of a 
rotary converter. 

Reference. Page 318. 

Connection. — On the A.C. end the machine is connected in the 
same way as a synchronous motor, see experiment 17, while on the 
D.C. end it is connected in the same way as a shunt generator. 



380 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xliii 

Method of Starting. — The machine may be started up as a 
motor from the D.C. end instead of by means of a starting motor, 
but it must be synchronized in the same way as a synchronous 
motor, see experiment 17, before it is connected to the A.C. 
mains. 

Readings. — With the applied voltage E 2 , the frequency, and 
E 3 It the output of the direct current side all constant, take 
readings of the current I and of the watts W for different values 
of the exciting current I f . Take a complete set of readings for 
1/4, 1/2, 3/4 and full load output from the direct current side. 

Curves. — Plot armature current, and power factor 

( — ,7 ) , on an exciting current base. 

worts amperes/ ' 

Plot applied voltage E 2 and direct voltage E z on a direct 
current (I ) base. 

Plot efficiency on a kilowatt output base at 100 per cent, and 
at some other power factor. 

Questions. — 1. Answer questions 1 and 3 in experiment 17 if 
that experiment was not performed. 

2. Why is the voltage of the direct current side independent 
of the field current, and how can this voltage be controlled? 

EXPERIMENT 19 

Object of Experiment. — To determine the starting and the 
running characteristics of a polyphase induction motor. 

References. Chaps. XXXVI and XXXVII, page 283. 

Connections. — The student should draw out a diagram of 
connections and specify the range of the instruments required. 

Readings. — To show the effect of increase in applied voltage 
on the current, torque and power factor at starting (start with 
a low value of applied voltage). 

Also readings to show the variation of efficiency, power factor, 
current, and speed with horse-power output, the applied voltage 
and frequency being normal and constant. 

Curves. — Draw the curves necessary to show the characteristics 
clearly. 

Questions. — 1. Why is the current for a given torque greater 
when the motor is at standstill than when running? 

2. What are the advantages and disadvantages of the squirrel 
cage induction motor compared with the wound rotor induction 
motor? 



Art. 404] 



LABORATORY COURSE 



381 



3. What are the relative advantages and disadvantages of 
the squirrel cage induction motor and the synchronous motor? 

4. Explain the shape of the power factor curve. Why is the 
power factor low at light loads? 

5. Explain the shape of the speed curve. 

6. How would you reverse the direction of rotation of an 
induction motor? 

7. How can the speed of an induction motor be varied for a 
given load? How does the induction motor compare with the 
direct current shunt motor for the operation of machine tools, 
and with the direct current series motor for the operation of 
cranes ? 

&, What effect has an increase or decrease of applied voltage 
on the speed of an induction motor at no-load and at full-load? 
Try this experiment. 



EXPERIMENT 20 

Object of Experiment. — To determine the voltage and current 
relations with different transformer connections. 

References.— Pages 233 to 238, also Chap. XXXV. 
Connections.— a Y to F, Fig. 322, page 276. 
b Y to delta 
c delta to delta 

d Scott connection to obtain two phase from 
3 phase. 



Fi S 




Note. — Each transformer should be protected with fuses in 
case of short circuit. If for example the third transformer in a 
delta connected bank was connected as in diagram B, Fig. 449, 
instead of as in diagram A, there would be a large voltage be- 
tween points Si and $ 3 , and if they were joined together a very 
large current would flow through the closed circuit. 

Readings. — Measure the voltage per phase, the voltage be- 
tween lines, the current in each transformer and the current in 



382 PRINCIPLES OF ELECTRICAL ENGINEERING [Chap, xlii 

the lines, and compare the values obtained with the theoretical 
values. 

Question. — 1. What are the advantages and disadvantages 
of the Y and delta connections? 

2. Show by experiment that three phase power can be ob- 
tained from a delta connected bank if one transformer is removed, 
but cannot be obtained from a Y- connected bank after the 
removal of one transformer. 



INDEX 



Adjustable speed operation, in- 
duction motors, 293, 298, 
300 

series motors, 92 

shunt motors, 89, 105-112 
Air blast transformers, 271 
Air compressors, drive for, 296 
Air gap, direct current machines, 48, 
68 

induction motors, 292 
All day efficiency, 269 
Alternating current, 48, 197 
Alternator, armature reaction of, 
244 

characteristics, 244 

construction, 229, 239 

efficiency, 250 

excitation, 240 

inductor type, 241 

magneto, 242 

parallel operation of, 259 

rating, 251 

reactance, 246 

regulation, 245, 248 

revolving armature type 240 

revolving field type 229, 239 

simple, 191 

single-phase, 231 

three-phase, 231 

two-phase, 230 

vector diagram, 245 
Aluminium arrester, 344 
Amalgamation of zinc, 143 
Ammeter, 8, 19, 198 
Ammeter shunt, 368 
Ampere, 5, 18 
Ampere-hour, 18, 33 
Ampere-hour efficiency, 153, 161 
Ampere-turn, 33 
Anode, 139 
Arc lamps, 354, 365 
Arc light generator, 75 



Arc welding generator, 190 
Armature, 49 58, 
Armature copper loss, 95 

core, 55 

reaction, 67, 83, 89, 244, 312 

resistance drop, 72, 79 

windings, 48, 230 
Arrester, lightning, 343 
Automatic current regulator, 76 

feeder regulator, 281 

motor starter, 130, 133, 306 

switch, 183 

voltage regulator, 74, 249 
Automobile batteries, 149, 151, 153 

lighting, 186 
Autotransformer, 279, 301 
Average current, 197 
Axle generator, 182 

Back e.m.f., 79, 253, 261, 311 
Balanced load, 238, 316 
Balancer, 316 
Battery capacity, 153, 155, 175 

characteristics, 152, 160 

construction, 148, 158 

control, 171-179 

efficiency, 153, 161 

electromotive force, 141, 142, 
151, 161 

primary, 140-145 

resistance, 141, 152, 161 

storage, 146-161 

temperature, 155, 161 
Blow-out coil, 115, 123, 129 
Booster, 173, 176, 180, 315 
Boosting transformer, 281 
Boring mill, motor for, 108 
Brake, 46, 333 
Brake test, 98 
Braking, dynamic, 333 
Brushes, 50, 58 
Brushes, carbon, 66 



383 



384 



INDEX 



Brushes, resistance of, 63, 66 

shifting of, 63, 83 
Bucking field coils, 186 
Building up of voltage, 71 

C.A.V. generator, 189 

Cables, underground, 347 

Candle-power, 354, 365 

Capacity circuits, 218 

Capacity of a battery, 153, 155, 175 

Capacity reactance, 221 

Car lighting, 181 

Carbon battery regulator, 176 

brushes, 66 

incandescent lamp, 352 

lamp regulator, 170, 182 

pile rheostat, 29 

switch contacts, 114 
Cast iron grid resistance, 27 
Cement mill motors, 297 
Characteristics, alternator, 244 

battery, 152, 160 

d.c. generator, 70-77 

d.c. motor, 85-94 

induction motor, 290, 292 

single-phase motor 310, 311 
Charge of a condenser, 218 
Choke coil, 343 
Circuit, breaker, 39, 117, 170 

formulae, 223 

magnetic, 33, 46 
Circuits, electric and hydraulic, 18 

parallel and series, 22, 212, 216, 
224, 226 
Circular mil, 20 
Circulating oil method of cooling, 

272 ' 
Clutch, electromagnetic, 46 
Coefficient of adhesion, 323 

of self induction, 206 
Color of light, 362 
Commutation, 62, 68, 83 

limit of output, 100 

of a.c. motors, 314 
Commutator, 50, 58 
Compensating winding, 312 
Compensator, starting, 302 
Compound excitation, 61 

generator, 74, 166 



Compound motor, 93 

starter, 120 
Condensers, 218, 222 
Constant current generator, 75 

regulator, 76 

transformer, 267 
Contactor switch, 116, 132 
Contacts, carbon, 114 
Control, of batteries, 171-179 

of railway motors, 126, 133, 327 

multiple unit, 133 

series parallel, 126 
Controllers, 122, 124 

automatic, 130 

crane, 122 

magnetic switch, 131 

master, 132 
Conversion factors, 15 
Converter, mercury vapor, 357 

rotary, 318 
Cooling of machines, 99, 100 

transformers, 270 
Copper losses, 95 
Core depth, 49, 56 

laminations, 55 
Core type transformers, 279 
Corkscrew law, 5 % 
Cost of motors, 101, 108 
Coulomb, 18 
Coulomb's law, 1 
Counter e.m.f., 79 
Crane motors, 92, 103, 298, 332 
Cross magnetizing effect, 67, 83, 312 
Current, alternating, 48, 197, 212 

direct, 48 

direction of, 4 

effective, 197, 212 

unit, 5 
Current carrying capacity of wires, 

368 
Current transformer, 351 
Cycle, 194 

Dampers, 295 

Daniell cell, 141 

Delta connection, 233, 236, 276, 278, 

Demagnetizing effect, 68, 83 

Dielectric strength, 22 

Differential booster, 176 



INDEX 



385 



Direct current, 48 

Direct current generator, see Gene- 
rators 
Direction of current, 4 

e.m.f., 9, 11 

force on a conductor, 7 

magnetic field, 2 
Disconnecting switch, 345 
Driving force of a motor, 78 
Drop in armature, 72, 79 

in transmission lines, 23 
Drum type controllers, 124 

windings, 52 
Dry cells, 143 
Dynamic braking, 333 

Eddy current loss, 96, 269 
Edison battery, 157 
Edison Lalande cell, 144 
Effective current, 197, 212 
Efficiency, 97 

alternator, 250 

battery, 152, 161 

direct-current machines, 98 

illuminants, 360 

induction motor, 299 

rotary converter, 320 

transformer, 268 

values of, 98 
Electric furnace, 74 

hammer, 38 

locomotive, 332 

welder, 264 
Electrical degrees, 200 

energy, 14 

power, 14 
Electrodynamometer instruments, 

199 
Electrolysis, 139 
Electrolyte, 139, 156, 160 
Electromagnet, 5, 37 
Electromagnetic brakes, 46, 333 

clutches, 46 

induction, 9 

motor, 41 
Electromotive force, back, 79 

direction of, 9, 11 

generation of, 9, 11 

of a battery, 141, 142, 151, 161 



Electromotive force, of self induc- 
tion, 12, 
unit of, 9 

Elevator motors, 103 

Enclosed arc lamps, 355 

Enclosed machines, 100 

End cell control of batteries, 172 

Energy, chemical and electrical, 140 
heat and electrical, 15 
mechanical and electrical, 14 
required for an electric car, 326 

Equalizer connection, 166 

Ewing's theory of magnetism, 36 

Excitation, 59, 71 

Exciter for alternators, 240 

Exciting current, 60 

External characteristics, 71 

Fan drive, 111, 112, 296. 300 

Farads, 219 

Farm house lighting, 169 

Feeder regulator, 281, 339 

Field copper loss, 95 

Field, magnetic, 1-5, 32 

Field, reversing, 65, 68, 83 

Field rheostat, 74 

Flame arc lamp, 356 

Flashing at switches, 12 

Flat compounding, 75 

Fleming's rule, 9 

Float switch control, 130 

Floating battery, 154, 179 

Flux density, 3, 33, 44 

Flywheel motor generator set, 335 

Flywheel, use of, 94, 104 

Force on a conductor, 6 

Formulae for circuits, 223 

Frequency, 194, 341 

natural, 226 

of resonance, 226 
Frequency changer, 321 

meter, 195 
Furnace, induction, 263, 267 
Fuses, 117 

Gas-filled lamp, 353 
Gassing of batteries, 147 
Generator, axle driven, 182 

car lighting, 181 

constant current, 75 



386 



INDEX 



Generator, direct-current, armature 
reaction, 67 
characteristics, 71 
commutation, 62, 68 
compound, 74 
construction, 56 
excitation, 60 
limits of output, 100 
parallel operation, 164, 166 
regulation, 71 
series, 75 
shunt, 72 
windings, 48 
induction, 294 
retarding force of, 78 
three-wire, 317 
variable speed, 181 
Glare, 363 

Gramme ring winding, 48 
Grid resistance, 27 
Growth of current, 12 

Hammer, electric, 38 

Head and end system, 181 

Heat and electrical energy, 15 

Heater units, 27 

Heating of machines, 99, 100, 239 

of transformers, 270 
Henry, 207 
Hoist motor, 332 
Hoisting, 332 
Holding magnet, 43 
Horn gap arrester, 345 

switch, 116 
Horse-power, 14 
Horse-power-hour, 15 
Hunting, 258, 295 
Hydrometer, 156 
Hysteresis, 36 

loss, 95, 269 



Induction, electromagnetic, 9 
furnace, 263, 267 
generator, 294 

motor, adjustable speed opera- 
tion, 293, 298 
applications, 296 
characteristics, 291 
construction, 283 
efficiency, 290, 299 
for railway service, 327 
power factor, 293 
single-phase, 308 
slip, 290 

speed, 287, 290, 293 
squirrel cage, 283, 296 
starters, 301-307 
starting torque, 287, 297 
vector diagram, 291 
wound rotor, 288, 297 
mutual, 11 
regulator, 281 
self, 11, 206 
Inductive circuit, 207, 210 

reactance, 208 
Inductor alternator, 241 
Instrument transformers, 351 
Insulating materials, 22, 26, 32, 99 
Insulation of windings, 58 
Insulators, line, 346 
Intensity of illumination, 364 

of magnetic field, 2, 3 
Intermittent ratings, 101 
Internal resistance of a battery, 141 , 

152, 161 
Interpole machines, 65, 100 
Ions, 139 
Iron loss, 96 

Iron, magnetic properties, 32 
Ironclad solenoids, 41 
Isolated lighting plants, 169 



Ignition, make and break, 206 
Illuminants, efficiency, 360 
Illumination, principles, 362 

intensity, 364 
Incandescent lamps, 352 
Inductance, 205, 209 

adjustable, 210 

of transmission line, 211, 216 



Joules, 14 



Kathode, 139 
Kilovolt-ampere, .251 
Kilowatt, 14 
Kilowatt-hour, 15 
Knife switches, 114 



INDEX 



387 



Lagging current, 203, 208 
Lamination of armature core, 55 

transformer core, 269 
Lamps, arc, 354 

connection of, 365 

incandescent, 352 

mercury vapor, 359 
Lamp circuit regulator, 170, 182 
Lathes, drive for, 108, 299 
Lead battery, 146 
Leading current, 203, 221 
Leakage reactance of transformers, 

264 
Leclanche cell, 143 
Left hand rule, 7 
Lenz's law, 10 , 
Lifting magnets, 43 
Light, quality of, 363 

unit of, 354 
Lighting of cars and vehicles, 181 

country roads, 363, 364 

drawing offices, 363 

farm houses, 169 

reading rooms, 362 

streets, 360, 364 
Lightning arresters, 343 

protection, 346 
Limits of output, 100, 255, 291 
Line construction, 346 
Line shaft drive, 102, 297 
Lines of force, 2, 3 
Liquid rheostat, 29, 335 
Loading back tests, 163 
Local action, 143 
Locomotive, electric, 332 
Long shunt, 61 
Losses in alternators, 250 

direct-current machines, 95. 

transformers, 268 

transmission lines, 215 
Lumens, 364 
Luminous arc lamps, 357 

Machine tool drive, 108, 299 
Magnet, 1 

alternating current, 306 

electro, 5, 37 

lifting, 43 

permanent, 35, 59 



Magnet, pull of, 40 
Magnetic, brakes, 46, 333 
circuit, 33, 46 
clutches, 46 
field, 1-5, 32 
flux density, 3, 33, 44 
hammer, 38 
properties of iron, 32 
separator, 47 
switch controller, 131 
Magnetism, molecular theory of, 36 

residual, 35, 70 
Magnetization curves, 34, 70 
Magnetizing current, induction 
motor, 292 
transformer, 262 
Magneto, 59, 242 
Magnetomotive force, 34 
Make and break ignition, 206 
Manganin, 21 
Manholes, 347 
Master controller, 132 
Maximum output of direct-current 
machines, 100 
induction motors, 291 
synchronous motors, 255 
Maxwells' formula, 44 
Mercury vapor converter, 357 

lamp, 359 
Mechanical losses, 95 
Mil, circular, 20 
Molecular magnets, 36 
Motor, direct-current, armature re- 
action, 83, 89 
commutation, 83 
compound, 93 
driving force, 78 
electromagnetic, 41 
limits of output, 100 
railway, 327 
series, 90 
shunt, 85 
speed, 80, 82 
starters for, 87, 117, 130 
theory of operation, 80 
variable speed, 89 
induction, see induction motor 
repulsion, 313 
single-phase induction, 308-310 



388 



INDEX 



Motor, single phase series, 310, 313, 
327 

synchronous, see synchronous 
motor 
Motor applications to air com- 
pressors, 296 

boring mills, 108 

cement mills, 297 

cranes, 92, 103, 298, 332 

crushers, 94 

elevators, 103 

fans, 111, 112, 296, 300 

lathes, 108, 299 

line shafts, 102, 297 

machine tools, 108, 300 

printing presses, 111, 113 

pumps, 103, 296 

shears and punch presses, 103, 
299 

textile machinery, 298 

wood-working machines, 102, 
297 
Motor car lighting, 186 
Motor car trains, 332 
Motor generator sets, 315, 335 
Multiple switch starter, 119, 13T, 
305 

unit control, 133 

voltage system, 109 
Multipolar machines, 51, 56, 287 
Mutual induction, 11 

Natural frequency, 226 

Neutral line, 62 

No-load saturation curve, 70 

No-voltage release, 87, 125, 302, 304 

Non-arcing metal, 343 

Non-inductive resistance, 211 

Ohm's law, 20 

Oil circulation for transformers, 272 

Oil for transformers, 270 

Oil switch, 300, 345 

Open delta connection, 277 

Open machines, 100 

Oscillograph, 193 

Overcompound generator, 75 

Overhead line construction, 346 



Overload relay on oil. switch, 345, 
350 
release, 118, 125, 304 

Parallel circuits, 22, 216, 226 

Parallel operation of alternators, 259 
direct-current generators, 164, 

166 
rotary converter, 320 

Pasted plates for a battery, 149 

Permanent magnets, 35, 59 

Permeability, 33 

Phase, relation, 203 

single-, two-, and three-, 231 

Pin insulators, 346 

Plant e plates, 148 6 

Polarization, 141 

Polyphase circuits, 238 
rotary converter, 320 

Potential transformer, 351 

Power, 14 

in a capacity circuit, 223 
in an inductive circuit, 209 
in a resistance circuit, 21, 211 
in a three-phase circuit, 236, 237 
measurement of, 214, 238 

Power factor, 213 

correction, 256, 320 
of an induction furnace, 267 
of an induction motor, 293 
of a synchronous motor, 256 
of a transformer, 262 

Power station, 171, 338 

Primary of a transformer, 261 

Printing press drive, 111, 113 

Pull of magnets, 40, 43 
' solenoids, 37 

Pump drive, 103, 296 

Punch press drive, 103, 299 

Puncture of insulation, 22 

Quantity of electricity, 18, 218 
Quick break switch, 114, 300 

Railway motors, 327 
Rating of machines, 101, 251 
Ratio of transformation, 262 
of rotary converter, 318 



INDEX 



389 



Reactance, capacity, 221 

inductive, 208 

of an alternator, 246 

of a transformer, 264 

of a transmission line, 211, 
216 
Reflectors, 359 
Regulation curves, 71 

of alternators, 245, 248 

of direct-current generators, 
71-75 

of a transmission line, 215 

speed, 107, 113, 293 
Regulator, axle generator, 184, 187 

constant current, 76 

feeder, 281, 339 

lamp circuit, 170, 182 

voltage, 74, 249 
Reluctance, 33 
Repulsion motor, 313 
Residual magnetism, 35, 70 
Resistance, 19 

battery internal, 141, 152, 161 

brush contact, 63, 66 

control of batteries, 171 

drop in armature, 72, 79 

for adjustable speed motor, 
112, 300 

power loss in, 21, 211 

specific, 20 

.starting for motor, 85, 87, 89, 
118, 305 

steadying, 365 

temperature coefficient of, 21 
Resistors, 26 
Resonance, 224, 227 
Reverse current circuit breaker, 170 
Reversing drum, 129 
Reversing field, 65, 68, 83 
Revolving field alternator, 192, 229 
239 

of an induction motor, 284 
Rheostats, 25 

carbon pile, 29 

cast-iron grid, 27 

field circuit, 74 

liquid, 29, 335 
Right hand rule, 9, 192 
Ring winding, 48 



Rosenberg machine ,188 
Rotary converter, 318 
Rotor, 229 

induction motor, 283, 288 

Safety devices, 337 
Saturation curve, 70 

magnetic, 46, 71 
Scott connection, 274 
Searchlight generator, 190 
Secondary of a transformer, 261 
Self-cooled transformer, 270 
Self excitation, 60, 71 
Self induction, 11, 206 
Self-starting synchronous motor 

294, 296 
Semi-enclosed machines, 100 
Separately excited machines, 60, 7-1 
Separator, magnetic, 47 
Series arc lighting, 366 

booster, 180 

circuits, 22, 212, 224 

excitation, 60 

generators, 75 

motors, 90, 310 

parallel control, 126 
• shunt, 77, 167 
Shades for lamps, 359 
Shadows, 363 

Shears, motor for, 103, 299 
Shell type transformers, 279 
Shifting of brushes, 63, 83 
Short-circuit curve, 246 
Short shunt, 61 
Shunt excitation, 60 

generators, 72, 162, 164 

motors, 85, 105 
Single-phase, 231 

motors, 308-314 

railway system, 329 
Sliding contact starter, 117 
Slip of induction motors, 290, 293 
Slipping belt generator, 185 

clutch generator, 186 
Slow speed drive, 113 
Solenoid, 32, 37, 41 

brake, 333 

starter, 130, 306 
Sparking, at a commutator, 63 



390 



INDEX 



Sparking, at a switch, 12 

limit of output, 100 
Specific gravity of electrolyte, 156, 
157 

inductive capacity, 219 

resistance, 20 
Speed, adjustable, 89, 92, 105-112, 

293, 298, 300 
Speed of a motor, 80-83, 89, 101 

of an induction motor, 287, 290, 

293, 298 

of a series motor, 92, 312 

of a shunt motor, 89, 105-112 

of a synchronous motor, 252, 
296 

regulation, 107, 113, 293 

regulators, 121 

synchronous, 252 

variation by armature control, 
105, 112 
by field control, 107 
Speed time curve, 323 
Spider, 59 

Split-phase method of starting, 308 
Split-pole rotary converter, 320 
Squirrel-cage motor, 283, 296 
Star-delta starter, 304 
Starters, automatic, 130, 133, 306 

hand-operated, 87, 117-121, 
301-305, 340 
Starting compensator, 302 
Starting torque of an induction 
motor, 287, 297 

of a series motor, 90 

of a shunt motor, 85 

of a synchronous motor, 252, 

294, 296 

Starting resistance, 85, 87, 89, 118, 

305 
Stator, 229, 283 
Steadying resistance, 365 
Stiff field, 69 
Stone generator, 185 
Storage battery, see battery, 146- 

161 
Stray loss, 96 
Street car controller, 126 
Street lighting, 360, 364 
Substation, 339 



Sulphation, 147 

Sum of alternating voltages, 203 
Suspension insulator, 346 
Switches, automatic battery, 183 

carbon break, 114 

contactor, 116, 132 

disconnecting, 345 

float, 130 

magnetic, 131 

oil, 300, 345 

quick-break, 114 

sparking at, 12 
Switchboards, 348 

Symbols for alternating currents, 198 
Synchronizing, 258, 378 
Synchronous motor, 252-259, 294- 
296 

applications, 296 

dampers for, 295 

hunting, 258, 295 

power factor, 256 

self-starting, 294, 296 

speed, 252, 296 

starting torque, 252, 294, 296 

synchronizing, 258, 378 

vector diagram, 254 
Synchronous speed, 252 
Synchroscope, 258, 350 

Temperature coefficient of resistance, 
21 
of a battery, 155, 161 
of motors, 99, 100, 102 

Terminal station, 339 

Textile mill, drive, 298 

Three-phase alternator, 231 
connection of load, 237 
connection of transformers, 276 
transformers, 279 

Three-wire generators, 317 
system, 316, 338 

Torque of a motor, 82 

of an induction motor, 287, 297 
of a series motor, 90, 310 
of a single-phase motor, 308, 310 
of a shunt motor, 85, 88 
of a synchronous motor, 252, 
294, 296 

Traction, 322-332 



INDEX 



391 



Tractive effort, 322 
Train lighting, 181 

friction, 322 
Transmission line, drop, 23 

inductance, 211, 216 

insulation, 346 

losses, 215 

regulation, 215 

single- and three-phase, 341 
Transmission, underground, 347 

voltages, 338, 341 
Transformation ratio, 262 
Transformer, boosting, 281 

connections, 273-279, 381 

constant current, 267 

cooling, 270 

efficiency, 268 

instrument, 351 

leakage reactance, 264 

lighting, 273 

losses, 268 

theory of operation, 261 

vector diagram, 263, 265 
Trolley wire, 329 
Tungsten lamp, 352 
Two-phase alternator, 230 

connection of transformers, 
273 

Underground cables, 347 
Unit of current, 5 

e.m.f., 9 

light, 354 

pole strength, 1 

power, 14 

work, 14 

V-connection, 277 
Variable speed generators, 182 
Variable speed operation, induction 
motors, 293, 298, 300 

series motors, 92 

shunt motors, 89, 105-112 
Vector diagram for alternators, 245 

induction motors, 291 



Vector diagram for parallel circuit, 
217, 227 

series circuit, 213, 225 

series motor, 311 

synchronous motor, 254 

transformer, 263, 265 

transmission line, 215, 342 
Vector sum, 203 
Vector representation, 201 
Vehicle lighting, 181 
Vibrating contact regulator, 187 
Volt, 9 

Volt efficiency, 152, 161 
Voltage, average, 197 

drop in transmission lines, 23 

effective, 212 

for arc lamps, 367 

for traction, 329 

for transmission, 338, 341 

of a battery, 151, 160 

regulators, 74, 249 
Voltages used in practice, 341 
Voltameter, 139 
Voltmeter, 19, 198 

Wagner motor, 314 

Ward Leonard system, 110, 335 

Water cooled transformers, 271 

Water rheostat, 29, 335 

Watts, 14 

Wattmeter, 214 

Wave form, 193 

Welder, electric, 264 

Windings, 48, 229 

Wire, current carrying capacity, 

338, 348, 368 
Wood-working machinery, motors 

for, 102, 297 
Work, unit of, 14 
Wound rotor motor, 288 

Y-connection, 233, 235, 237, 276, 

278 
Y-delta starter, 304 
Yoke of machine, 56, 58 












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